Abstracts most of the input mechanisms under an InputSubsystem class
that is managed by the frontends, eliminating any static constructors
and destructors. This gets rid of global accessor functions and also
allows the frontends to have a more fine-grained control over the
lifecycle of the input subsystem.
This also makes it explicit which interfaces rely on the input subsystem
instead of making it opaque in the interface functions. All that remains
to migrate over is the factories, which can be done in a separate
change.
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.
