Let players remap hotkeys in-game, fix default hotkeys being qwerty-specific.
- Provide a "Hotkey" screen to let players remap hotkeys in-game using a convenient setup.
- Make all .cfg hotkeys refer to scancodes (i.e. position on the keyboard), so that default hotkeys now translate correctly for AZERTY, QWERTZ and other layouts.
- 'BackSpace' is now an alias for 'Delete', and works for killing units. This fixes #1917, as macs don't have a proper delete key and would need to use Fn+Del otherwise. This shifts "timewarp" to Shift+BackSpace.
Functionally, this switches hotkeys to scancodes, as that makes more sense (they are combinations of key positions, not actual text output).
SDL includes are cleaned and key names are reused.
Fixes #2850, Fixes #2604, Refs #1810, Fixes #1917.
Follows work in rP19624.
Various diffs tested by: Angen, Stan, Freagarach
Comments by: Nescio
Differential Revision: https://code.wildfiregames.com/D2814