Allow hotkeys/config settings to be empty.

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