overview: Hide search results while leaving overview
There is currently no proper transition from search results to the session: Only the top bar fades, but the rest of the screen just changes abruptly at the end of the transition.
Fix this by hiding search results before leaving the overview, so the regular transition can take place.
Close #3821 (closed)
Copy of !1752 (closed), but using a branch name other than "master", so we don't try to build against an ancient version of mutter.