Fix plot widget drag and drop bug (Issue #716)

[Bartimaeus-]

• Plot widgets were not clearing their drag and drop state if a drag event left the widget, causing mouse events to not be handled properly if the drop event happened in another plot widget
• Added event handlers for drag leave event

(This was actually a pain to debug at first because it seemed like it was only impacting the first plot widget that existed, but it was just unfortunate that I was really repeatably dragging across the same widget)

[facontidavide]

fantastic, thanks!