qt: prepare hotkey editor for prefs redesign

This is another foundational chunk of work upon which I'm building a redesign for the prefs interface.

This focuses upon preparing the hotkey editor for being moved out of the simple and advanced general preference interfaces. I intend ultimately for there to be a sidebar giving access to the three general preference interfaces, the hotkey editor and the ML manager, along with an entry for the reset-all function.

This includes untangling the hotkey editor from ConfigControl, moving it to its own files, and converting it to be .ui based.

Note that to make things much easier, and because the hotkey editor doesn't fit very well in advanced view anyway, it is removed from there as an initial step, as discussed/justified in the commit log.