viewSelector: Cancel search on overview hidden

Currently when the overview is hidden, any pending search is kept alive,
not only at remote search provider level (as per issue #183), but even
the shell providers proxies continue to get and process data. This happens
even if this is not needed anymore, while the UI reset is performed only
next time that the overview is shown (causing some more computation
presentation time).

In order to stop this to happen, when the overview is hidden, we have to
unset the search entry to an empty value as this would make SearchResults
to have empty terms list and that would make the proxies cancellable to
be triggered (without causing any further search to start).

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/205

js/ui/viewSelector.js

@@ -311,6 +311,7 @@ var ViewSelector = new Lang.Class({
     },
 
     hide() {
+        this.reset();
         this._workspacesDisplay.hide();
     },