Use old layout code for non-Gtk3

[VisualMelon]

Fixes issues introduced with #12 not compiling with GTK#. Uses the old layout code for non-Gtk3.

Tested GTK#3 bits with the NET Core branch; hoping AppVeyor will be happy; will try to test on my old Win7 box.

Changes:

• introduce GTKSHARP3 macro for the GtkSharp3 project
• use new Layout code (from memory leak fixed(?) #12) in GtkSharp3 project, otherwise use the old code

[VisualMelon]

OK, old Win7 box still works, and the GTK# provision doesn't seem to be leaking.

[d-buchmann]

Sorry for introducing the hassle.
Honestly, I don't have any experience with CI and couldn't test with Gtk2.
I've already put some effort into building 2.12.45 from source, in vain however.

[VisualMelon]

@d-buchmann no need to apologise; after seeing what https://github.com/oxyplot/oxyplot-gtksharp/blob/master/appveyor.yml does, I installed a (hopefully corresponding) GTK on an old box with Chocolatey.

Are you able to check that this PR works for you with GTK#3?

[d-buchmann]

I'll try and check tomorrow. So long :)

[d-buchmann]

Tested ExampleBrowserGtk3 on Win10 and Linux successfully (only the PlotController examples actually, but if think it will suffice).

[VisualMelon]

@d-buchmann thanks!