search: Cancel search provider operations on clear

Ensure that the search provider operations (just getResultMetas requests
in the current implementation) in progress are properly cancelled when we
clear the UI, otherwise returned results might still be added when not
needed.

This is triggered for each provider by the SearchResults reset.

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

js/ui/search.js

@@ -192,6 +192,7 @@ var SearchResultsBase = new Lang.Class({
     },
 
     clear() {
+        this._cancellable.cancel();
         for (let resultId in this._resultDisplays)
             this._resultDisplays[resultId].actor.destroy();
         this._resultDisplays = {};