Add event hooks to clearing

Trigger events open.fndtn.clearing, opened.fndtn.clearing,
change.fndtn.clearing, close.fndtn.clearing, closed.fndtn.clearing at
appropriate spots.
Add documentation for clearing events

Docs and examples for open.fndtn.clearing, opened.fndtn.clearing,
close.fndtn.clearing, closed.fndtn.clearing and change.fndtn.clearing
Change event names in doc to match event names in code.
Add new event resized.fndtn.clearing

This event occurs after an image has been resized, so you can access
the new image size.
Merge pull request #3220 from anitagraham/add-events-to-settings

Add events to clearing