Code review comment for lp:~ntnk/gala/modifiers-only-input-source-switch

I think it will be better to just replace handle_switch_input_source() like this http://paste.debian.net/739520/, if we leave both then we will have conflicts when editing the 'current' key.

This will remove the possibility of configuring this keybinding through the Shortcuts tab on the keyboard plug, but I think no one really cares for non modifier-only keybindings for layout switching, except for our default Alt+Space which is still available through grp:alt_space_toggle.