overview: Add API to remove search providers

As extensions are now expected to provide a "disable" function,
they need to remove search providers they added. Implement the
removal functionality and add a public removeSearchProvider()
method.

https://bugzilla.gnome.org/show_bug.cgi?id=657548

js/ui/search.js

@@ -356,6 +356,14 @@ SearchSystem.prototype = {
         this._providers.push(provider);
     },
 
+    unregisterProvider: function (provider) {
+        let index = this._providers.indexOf(provider);
+        if (index == -1)
+            return;
+        provider.searchSystem = null;
+        this._providers.splice(index, 1);
+    },
+
     getProviders: function() {
         return this._providers;
     },