Fix memory leak in non-multi-process Firefox

Fix memory leak in non-multi-process Firefox

Apparently, 'overflow' and 'underflow' events in web page content can be caught
in UI listeners in non-multi-process. `event.target` of 'overflow' events in the
browser UI are saved as scrollable elements, which accidentaly included web page
elements as well. Holding on to such elements after their owner tab is closed
caused a memory leak (ghost windows). This commit checks that the overflown
elements really are UI elements before saving them.

Fixes #664.