Fix provided by NightStrike from mingw-w64 project

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...