Make cursoring over a FocusableNode work again

When you cursor onto a FocusableNode, it's selected, and we focus the
paste target as part of our hack to make copying FocusableNodes work
in Firefox. But then when you press the arrow key again, that event
isn't picked up by anything, and you can't move the cursor off the
FocusableNode using the keyboard.

Fixed by attaching the EventSequencer to this.$ (which is the parent
of $documentNode and $pasteTarget) and listening for focus/blur on
both $documentNode and $pasteTarget.

Bug: 54443
Change-Id: I7bddcfa9fa6f38908e315c97623bd27133daa98d