Update context menu according to stricter rules

* Connect to onContextChange instead of onModelUpdate
* Never emit context changes while staging

Change-Id: Id5d1dbb367d6e4e34d70bc8dd668b08bf928e4cd
Ignore staging events in CE surface

This prevents the document from stealing focus while live
editing a node.

Bug: 67326
Change-Id: If03a200ceadf16adffea497ca511d49bc687c31f
Merge "Update context menu according to stricter rules"
Merge "Ignore staging events in CE surface"