This covers the settings dialog itself and option pages based on
`UiFileBasedOptionPage`. Other option pages need to react to the
new `OptionPageWidget::retranslationRequired()` signal or handle
the `QEvent::LanguageChange` event which is now propageted to the
page's widget.
This makes the annoying hasBeenShown() check in each apply()
and reset() implementation unnecessary. It should not break
anything because if the page hasn't been shown there is nothing
to reset/apply anyways.
Martchus