[GActs] Invoke previous event filter in bundled Qxt copy.

For #1442.
[Util] Use QAbstractEventDispatcher::setEventFilter() for #1442.

qt_installX11EventFilter() is removed in Qt 5.
[KBSwitch] Use QAbstractEventDispatcher::setEventFilter() for #1442.

qt_installX11EventFilter() is removed in Qt 5.
[MT] Use QAbstractEventDispatcher::setEventFilter().

qt_installX11EventFilter() is removed in Qt 5.

This finally closes #1442.