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. This allows EDuke32 to run with wayland 🎉.

It can be controlled via gamepad keyboard or mouse which improves the experience on handhelds.

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

Future tasks

• The default ImGui isn't great for accessibility, support for diverse DPIs would be nice. Maybe once the dust has settled on https://github.com/ocornut/imgui/issues/8465
• Bring back the blue EDuke32 sidebar image using https://github.com/ocornut/imgui/wiki/Image-Loading-and-Displaying-Examples
• ImGui startwin for VoidSW etc.