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.
Follows work in rP19624.
Various diffs tested by: Angen, Stan, Freagarach
Comments by: Nescio
Differential Revision: https://code.wildfiregames.com/D2814