viewSelector: don't process key presses from capture-event

Rather than connecting to stage::capture-event and then trying to
guess whether or not a given key-press should be handled by us or not,
handle the end-search-on-Escape case from entry::key-press-event
(since it only makes sense when the entry is focused anyway) and the
start-search-on-printable-key case from stage::key-press-event (which
will only get the events that no other actor wanted for itself).

Similarly, do exit-overview-on-Escape and switch-panes cases from
stage::key-press-event, rather than
viewSelector.actor::key-press-event, so that they will work correctly
even if the keyboard focus is somewhere else. (Also fix a longstanding
bug in the pane-switching code, which was supposed to be disabled when
a search was active, but was checking a non-existent variable.)

https://bugzilla.gnome.org/show_bug.cgi?id=642502

js/ui/overview.js

@@ -513,7 +513,7 @@ Overview.prototype = {
         if (this._shown)
             return;
         // Do this manually instead of using _syncInputMode, to handle failure
-        if (!Main.pushModal(this.viewSelector.actor))
+        if (!Main.pushModal(this._group))
             return;
         this._modal = true;
         this._animateVisible();