Added --quit and --build-solutions cmd line options to make CI pipelines possible

[NathanWarden]

This adds the ability to use the following command line arguments:

-q/--quit
--build-solutions

-q/--quit auto quits the editor after one iteration through the main loop. This allows for assets to be imported and any other command line arguments to be run without hackish solutions such as @brainsick and I were discussing on this thread: #15957

The --build-solutions flag allows for C# to be built (and any other language someone might implement).

Both of these combined make it very easy to integrate and create a CI solution for game projects.

An example of how this looks in bash is:

$GODOT=/path/to/godot
$GODOT -e -q --build-solutions
$GODOT TestRunner.tscn

[brainsick]

You made that look downright easy! You didn't even need an alternative. LOL.

[NathanWarden]

@brainsick Thanks, I suppose if this PR is accepted I'll take a look and see if I can get this thing running headless next. There's a --no-window flag, but it looks like it only works on Windows.

[NathanWarden]

Okay, I updated this pr to call build for the solutions outside of the main loop so as to not have an extra if statement in the main loop.

edit: Put it back in the main loop since it was showing various errors (probably related to the progress bar).

[akien-mga]

Shouldn't it be ||?

[NathanWarden]

Yep, I'm not sure why I did that. Should be logical instead of bitwise now :)

[akien-mga]

I guess this one should also be enclosed in #ifdef TOOLS_ENABLED.

[NathanWarden]

I'd be happy to add it. The main reason I didn't is because there aren't any other command line flags, including for --editor, --project-manager, etc, that have the TOOLS_ENABLED compiler flag. Up to you :)

[akien-mga]

Actually if you rebase, they should have by now :)

[NathanWarden]

Cool, thanks for letting me know. They're now moved into the compiler flag :)

[akien-mga]

Thanks!