Build Surge with a fork of VSTGUI (#527)

This diff begins building Surge with a fork of VSTGUI rather than the
VSTGUI which ships with the vst3sdk pointer. It accomplishes it by the following:

1. there is a fork of vstgui called `surge-synthesizer/vstgui` with a branch
called `surge` which is master + diffs
2. There is a submodule in surge called `vstgui.surge` which points at that
branch
3. There are changes to premake to point our vstgui implementation

This version points to a vstgui which integrates three of @sagantech's
changes

Closes #394: Cannot move window after selecting preset
Closes #396: Diacritical marks in menus
Changes for submenu checkmarks on windows

Also this diff implements our fork strategy described in #515 and
closes #515 as a result.

.gitmodules

@@ -1,3 +1,7 @@
 [submodule "vst3sdk"]
 	path = vst3sdk
 	url = https://github.com/steinbergmedia/vst3sdk.git
+[submodule "vstgui.surge"]
+	path = vstgui.surge
+	url = https://github.com/surge-synthesizer/vstgui.git
+	branch = surge

doc/vstgui-dev.md

@@ -0,0 +1,84 @@
+# Developing with our VSTGUI Fork
+
+## Motivation 
+
+We made the hard choice [here](https://github.com/surge-synthesizer/surge/issues/515) to fork
+vstgui so we can maintain our own fixes while also submitting them to steinberg. That issue describes
+a lot of the conversation, but our key motivation was
+
+* We wish to track the master version of vstgui closely
+* We need to fix bugs in vstgui which expose Surge to odd behaviors
+* We are happy to fix those bugs and send those changes to the vstgui team, but would like to ship Surge with
+the fixes without having to wait for the vstgui team to review and merge them, since their current cadence is different
+than our current cadence.
+
+So we decided to take make our own fork of vstgui, have a branch of our own which is branched
+from master, add patches to our branch, and submit those patches as PRs to steinberg.
+On some regular basis we will rebase our branch to the latest steinberg master.
+
+## How it is set up
+
+* [`surge-synthesizer/vstgui`](https://github.com/surge-synthesizer/vstgui) is a fork 
+of [`steinbergmedia/vstgui`](https://github.com/steinbergmedia/vstgui) with default branch set to master
+* [`surge-synthesizer/vstgui:surge`](https://github.com/surge-synthesizer/vstgui/tree/surge) is a branch 
+from [`surge-synthsizer/vstgui:master`](https://github.com/surge-synthesizer/vstgui/tree/master) with changes we have applied along the way
+* [`surge-synthesizer/surge`](https://github.com/surge-synthesizer/surge) has a submodule `vstgui.surge` which points to the `surge` branch of our `surge-synthesizer/vstgui` fork.
+* `surge-synthesizer/surge` premake uses the version of vstgui from the submodule `vstgui.surge` rather than the version from the submodule `vst3sdk/vstgui4/vstgui`
+
+This amounts to us having a repo which is ahead of `steinbergmedia/vstgui:master` while still having the master version of the rest of
+the `vst3sdk` components.
+
+Please note that `steinbergmedia/vstgui` has `develop` as its default branch. This means branches like our `surge` branch (which is a branch from `master`) and 
+steinberg's `master` branch will 
+report being many commits behind the repo in the github UI. We think it is prudent for surge to track a patcked master, not a patched develop, so have changed our default
+branch in our fork back to master.
+
+## How to develop
+
+There's a couple of ways for you to develop changes but the basic idea is as follows.
+This is a bit tricky. If you are doing this for the first time, probably best to ask
+on slack or IRC.
+
+* Fork your own copy of https://github.com/surge-synthesizer/vstgui or work directly in the repo. *Do not fork the 
+steinberg repository. You want a fork of our repository (so a fork-of-a-fork)*.
+* Clone either the repo or your fork on your filesystem and set the environment variable `VSTGUI_DIR` to point at it. Clean and run premake in `surge`.
+* Checkout the surge branch in your vstgui
+* From there, make a feature branch. So 'git checkout -b my-change' on top of 'surge' not 'master'
+* Make your change, commit, push as normal. Since VSTGUI_DIR points to your copy your builds will pick up your changes
+* At this point we want to do two things
+  * Take your branch as a PR to steinberg. Do this using standard methods.
+  * Integrate your branch into the surge repository and move the submodule pointer
+  * That last step is a bit tricky. Really best to ask on Slack or IRC if you are
+    doing this for the first time. Or just let one of the maintainers know and we
+    can do the git shenanigans.
+
+This documentation could be more fulsome obviously. Really the first person other than @baconpaul to
+go through the process, if they could document it, we can update this doc then.
+
+
+## Historical Record: How I set up the thing in the first place
+
+* fork steinbergmedia/vstgui into surge-synthesizer
+* set the default branch as "master"
+* git checkout -b surge
+* git push origin surge
+* modify premake to set up a vstguidir
+
+To apply my first diff
+
+* get a vstgui
+* git checkout surge
+* git checkout -b surge-menu
+* fix
+* git commit
+* git checkout surge
+* git merge surge-menu
+* git push
+
+Now add this as a submodule
+
+* git submodule add -b surge --name vstgui.surge https://github.com/surge-synthesizer/vstgui vstgui.surge
+
+And write this doc and we are done
+
+

premake5.lua

@@ -6,7 +6,14 @@ language "C++"
 
 -- GLOBAL STUFF --
 
-VSTGUI = "vst3sdk/vstgui4/vstgui/";
+local VSTGUI_DIR = os.getenv("VSTGUI_DIR")
+
+if VSTGUI_DIR then
+   VSTGUI = VSTGUI_DIR .. "/vstgui/"
+else
+   VSTGUI = "vstgui.surge/vstgui/"
+   -- VSTGUI = "vst3sdk/vstgui4/vstgui/";
+end
 
 defines
 {