Fix #2824, stop updating the tab state on tab 'updated' event
Fix #2824, stop updating the tab state on tab 'updated' event

Relocate the urlEnabled check to the button click handler, and show the
unshootable page error if the url isn't enabled.

Removing the onUpdated handler and the many redundant setIcon calls will
hopefully help fix performance issues currently preventing uplift into
Firefox.

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