Known issue: mouse hiding is not implemented (yet)

It does not seem to matter, but better safe than sorry...

This allows popup menu and fullscreen toggle to work outside of the
video rendering area.

Also emit layoutAboutToBeChanged/layoutChanged() more. Speeds up
playlist-view atleast for me

we don't use per-item showflags anyway, as currently they are all same
on model-wide. And I didn't comeup any use-case currently that would
need per-item showflags.

Set currentItem to NULL if there aren't anything playing on rebuild()