Fix fullscreen mode

In some setups, the FullScreen action would get out of sync with
the actual window state and thus kick Quassel into an endless loop
and finally a crash when toggling full screen mode.

Turns out reading the docs for KToggleFullScreenAction helped doing
this properly (avoiding this and other bugs when using QWidget's methods).

Thanks to yofel and hays for reporting, and apachelogger for putting
me on the right track for fixing this.

Fixes #1153, fixes #1258.