With Sven's Qt editor almost ready for merge, it's time to think about the future of the GTK platform. I based the Linux compatibility work on the SDL platform, which did not have an editor before and thus was easier to adjust. The result is that on the qteditor branch, neither GTK with or without the Qt editor currently build. I'd argue that we're better off removing GTK completely than making it work again:

- SDL is already a dependency for the controller support.

- There is no reason the keep the old editor interface as it's clearly inferior to the cross-platform Qt editor (this applies to all platforms).

- The GTK platform is a lot more complex than the SDL platform, with roughly double lines of code in C4App and C4Window (513 SLOC for SDL, 1027 for GTK. As comparison, Win32 has 851 SLOC and I'm not sure about Mac).

- SDL is a lot more common as a game development base than GTK.

- In the future, we may be able to remove more platforms by switching fullscreen to SDL or Qt for all operating systems (probably doesn't matter which as we won't be able to remove either dependency).

There's obviously also option c) keep the broken GTK code until someone bothers to fix it, but I don't think that's a good idea.

In case someone wonders about the stability of the previously-hidden SDL code: I've played exclusively with an SDL-based engine in the last few months. I had to fix a couple of issues, but it works fine now and apparently crashes less than the Windows engine ;)

Done: https://github.com/openclonk/openclonk/commit/b7359e0c27210b59969b5d05b184f04683cdf16c