Use GTK on window

[nico-abram]

In theory this would be as simple as:

• Link GTK on windows (Somehow, ideally add the source in /extern/ and link it with cmake, probably add a .cmake file in /extern/ for it and add it to the main cmake extern file, then maybe add some things in /src/CMakeLists.txt)
• Include the file in arch_default.h for windows
• Make LoadingWindowGTK.h try GTK for windows

Then, practically, this might require:

• Make the loading window use the new layout (full-window image)
• There might be non cross-platform code in the stepmania gtk window code. Make it portable.

This would allow us to nuke the windows loading window and the thing itself would probably be faster/ more efficient than our raw implementation.

[nico-abram]

Superseded by #1073