Bug 1093686 - Ignore body-level touch/mouse listeners when assessing element clickability. r=roc

When walking up the content tree to assess if an element is contained inside a
clickable element, we should stop before hitting the body element. This is a
heuristic based on pages in the wild, because a lot of pages have mouse/touch
listeners on the body. Without this patch all elements end up getting treated
as clickable which makes the event retargeting code useless.
Bug 1093686 - Add some debug logging code for PositionedEventTargeting.cpp. r=roc
Bug 1093686 - Add a test to ensure event listeners on the body don't affect event retargeting. r=roc