Rethink (refactor and possibly change) the old keybind and slash command handling

[rdw-software]

Might have to wait until after the config GUI is implemented, as slash commands would be less useful afterwards. They're really ugly though, and I never liked them.

• Evil purged from the Keybind handler (Controllers\Keybinds.lua)
• Exorcised all demons from the Slash command handler (Controllers\SlashCommands.lua)