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.
These slots are only ever attached to event handling mechanisms within
the class itself, they're never used externally. Because of this, we can
make the functions private.
This also removes redundant usages of the private access specifier.
