Update selectedAnnotations at annotationUpdate event

If we got annotations selected in the sidebar and delete one, the filter
doesn't get updated. Alas, if we delete the last selected annotation then
the sidebar will show zero annotations but the filter will still say:
"Showing x annotations..."

This fix gives a new listener to the annotationDeleted event and properly
updates the selected annotation dict and count if needed.

Fix #1591
Merge pull request #1635 from hypothesis/1591-clear-filter-if-selection-is-empty

Update selectedAnnotations at annotationUpdate event.