Our currently ad-hoc method of tracking element references is basically
a process-wide map, rather than grouping elements according to their
browsing context groups. This prevents us from recognizing when an
element reference is invalid due to its browsing context having been
closed.
This implements the WebDriver spec concept of element references grouped
according to their browsing context groups.
This patch is a bit noisy because we now need to plumb the current BC
through to the element reference AOs.
This implements execution of the pointer up, pointer down, and pointer
move actions.
This isn't 100% complete. Pointer move actions are supposed to break
the move into iterations over the specified duration, which we currently
do not do.