When embedded media in an annotation loads, this may trigger
a resize of the element's content and result in a change of whether
expand/collapse controls need to be shown, outside of Angular's
knowledge.

The DOM doesn't provide a nice way to monitor for resizing
of a specific elements (although hacks involving scroll events,
hidden <iframes> etc. exist) so here we just monitor for 'load'
events and trigger a digest in that case, as well as notifying
the containing directive whenever the collapsible state changes.