This changes ConfigDB to support empty settings, marked with `= ""`. It
also changes hotkey to handle unused hotkeys, but stil have them appear
in the hotkey editor.
Fixes an issue where the current editor would bug when saving an empty
hotkey.
Removes the old system for unused hotkeys, adjust hotkey files
accordingly.
Reported by: FeldFeld
Differential Revision: https://code.wildfiregames.com/D3307
This was SVN commit r24618.