Add comment regarding the removal of X11 hacks to GraphicalUi

Turns out all window activation stuff just works with Qt5, even with
KF5 integration enabled. So there is no need to employ fugly X11 hacks
in order to bypass focus stealing prevention, as was required in KDE4.

Because GraphicalUi doesn't seem to require any more porting changes,
add a comment instead to later remove the hacks together with Qt4/KDE4.