Customize hotkey categories & ordering, prepare for translation.

• adds JSON files to specify the human-readable categorization, name and description of hotkeys.
• clean up code

Translation support via messages.json not yet added - the strings need another pass and that avoids translators doing redundant work. See D3641

Refs #5867, Refs #5996

Differential Revision: https://code.wildfiregames.com/D3596