This will let us have a few extras widgets in the main dialog

( PLModel::childrenCount() > 0 ) is not equivalent to (
playlist_item_t*->i_children > -1 ). This patch swaps former for latter
which is a correct strict check wether an item is node.
Signed-off-by: Ilkka Ollakka <ileoo@videolan.org>

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