To prepare for translation support, this makes all of the widgets
cognizant of the language change event that occurs whenever
installTranslator() is called and automatically retranslates their text
where necessary.

This is important as calling the backing UI's retranslateUi() is often
not enough, particularly in cases where we add our own strings that
aren't controlled by it. In that case we need to manually refresh the
strings ourselves.

We don't need to extract the entire set of hotkeys into a list and then
iterate through it. We can traverse the list and early-exit if we're
able to.

1. This is something that should be solely emitted by the hotkey dialog
itself
2. This is functionally unused, given there's nothing listening for the
signal.

Avoids dumping all of the core settings machinery into whatever files
include this header. Nothing inside the header itself actually made use
of anything in settings.h anyways.

This doesn't actually modify instance state of the dialog, so this can
be made const.

* Adds a new Hotkeys tab in the Controls group.
* Double-click a Hotkey to rebind it.