Add support for asynchronous search providers

Some search providers may want to change their results, or may not
want to block on an external service to get their results (DBus, etc.)
Set up an infrastructure to allow search providers to add their search
results at a later time.

Based on a patch by Jasper St. Pierre and Seif Lotfy.

https://bugzilla.gnome.org/show_bug.cgi?id=655220

js/ui/viewSelector.js

@@ -297,7 +297,7 @@ SearchTab.prototype = {
     _doSearch: function () {
         this._searchTimeoutId = 0;
         let text = this._text.get_text().replace(/^\s+/g, '').replace(/\s+$/g, '');
-        this._searchResults.updateSearch(text);
+        this._searchResults.doSearch(text);
 
         return false;
     }