Check if node isn't collapsed before trying to move as a child
#210: add the ability to expand a node when hovering

Simply add "expand-on-hover" attribute which can be a boolean (true: expand
immediatly on hover, false: don't expand on hover) or a number (0: expand
immediatly, >0: wait for this milliseconds to expand).

Events are triggered on timeout start (expandTimeoutStart), cancel
(expandTimeoutCancel) and end (expandTimeoutEnd).