Integrate Website's Watched List

Refactor ChangeWatched to avoid duplication and extend it to update the
website's watched list when the watched status changes.
Add plugin menu option to browse the watched list and within that list
delete items.
Small fix to utils.py where xbmcgui.Dialog().yesno() didn't like being
passed None.
Merge remote-tracking branch 'origin/website-watched' into website-watched

Conflicts:
	pw_scraper.py
	service.py
	utils.py