Duke3D: Setup window using SDL2 and ImGui

gtk2 is deprecated #303. I considered porting it to gtk4 but I didn't feel like rewriting the dynamicgtk.h header.

This creates a setup window using SDL2 and ImGui then reuses that SDL2 window for the game.

It can be controlled via gamepad keyboard or mouse.

I may have to spend more time making sure it doesn't break other configurations.

I tested it with:

make -j 12 eduke32 FORCEDEBUG=1 LF= HAVE_GTK2=0