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()

Maybe this would actually works better if we store index, but maybe I'll
look it sometime later why it didn't work as I expected.

X does it automatically when the connection is closed

Deeper code can cope with too overly large values.
We need to accept seemingly too far start times because the length is
typically not known at startup (and hence set to zero).