[fix bug 981542] Add callback to the new close button in door-hanger

This PR does two things:

* Adds a callback to the new close (x) button in the door-hanger menu (use latest Nightly)
* Removes the initial blue UI highlight on the menu panel icon (required so [bug 971116](https://bugzilla.mozilla.org/show_bug.cgi?id=971116) can be landed).