Make worker non-inited at cleanup so that it will
queue messages until re-attached.
Detach worker if panel's contentURL is changed.
Remove unused event channnel.
Update getContentFrame function so that it returns own iframe when shown or showing.
destroy page-workers.
Update unload test.
Add test for Bug 921000
Merge pull request #1251 from Gozala/bug/panel-url-change@921000

Bug 921000 - Emits must be queued when page is loading r=@jsantell