Rectangle with fill is now added to drag and drop
Menu appears on right-click!
Added hardcoded tooltip on dragged Repr objects, hardcoded right
click menu.
Fixed right click menu positioning
Added delete action on right click to remove an item under cursor
- right click menu doesn't check if item is under it, so delete
  will always show. Deleting on empty space does nothing.
- callback for successful deletion of item added as kwarg.
  Defaults to print.