Merge branch 'sdl2-rebase' into 'master'

SDL2 migration

This branch contains the new code based on SDL2.

Some notes:

* Resolution detection code seems dubious and should be reworked
* I put the 60FPS code snippet found in the official SDL2 patch but I’m not sure what it does and how.
* Mousegrab needs to be fixed
* The format for key bindings in config.txt changed.
* All key detection is now based on scancodes and this may reveal unlogical under layout other than qwerty for things like meta+q, meta+g, and the debug keys.

This is intended to fix #2

See merge request !3