BUG ShowUnpublished checkbox with initial state not working

If ShowUnpublished checkbox initial state is checked, the history
doesn't show the unpublished versions up front. The user would need to
uncheck the box then check again, as only the change event would handle
the toggling of showing unpublished versions.

Moved the ShowUnpublished handler into a separate entwine class, and
run the same toggle code onmatch, as well as onchange.
Merge pull request #705 from halkyon/showunpublished_event

BUG ShowUnpublished checkbox with initial state not working