Firefox incorrectly goes back/forward in history instead of moving the cursor to
the beginning/end of the line when you press cmd-left/right on a mac. We now
override this to do the right thing.
When cleaning up pasted content, we remove any empty inline tags. However, we
should not be stripping <img> tags (which are of course both inline and empty).
Webkit needs a special placeholder text node as it can't focus empty text nodes.
This was being cleaned up too early, before the user had a chance to enter any
text.
