dash: Only create trash/mount apps if they are turned on

Previously, the apps always existed even if the icons were not shown.
This meant that the event handling was still being done wastefully.

So, let's only create the apps when they are actually needed.

dash.js

@@ -278,12 +278,6 @@ var MyDash = new Lang.Class({
 
         this._appSystem = Shell.AppSystem.get_default();
 
-        // Remove Drive Icons
-        this._removables = new Locations.Removables();
-
-        // Trash Icon
-        this._trash = new Locations.Trash();
-
         this._signalsHandler.add([
             this._appSystem,
             'installed-changed',
@@ -311,14 +305,6 @@ var MyDash = new Lang.Class({
             Main.overview,
             'item-drag-cancelled',
             Lang.bind(this, this._onDragCancelled)
-        ], [
-            this._trash,
-            'changed',
-            Lang.bind(this, this._queueRedisplay)
-        ], [
-            this._removables,
-            'changed',
-            Lang.bind(this, this._queueRedisplay)
         ]);
     },
 
@@ -763,11 +749,31 @@ var MyDash = new Lang.Class({
         }
 
         if (this._dtdSettings.get_boolean('show-mounts')) {
+            if (!this._removables) {
+                this._removables = new Locations.Removables();
+                this._signalsHandler.addWithLabel('show-mounts',
+                    [ this._removables,
+                      'changed',
+                      Lang.bind(this, this._queueRedisplay) ]);
+            }
             Array.prototype.push.apply(newApps, this._removables.getApps());
+        } else if (this._removables) {
+            this._signalsHandler.removeWithLabel('show-mounts');
+            this._removables = null;
         }
 
         if (this._dtdSettings.get_boolean('show-trash')) {
+            if (!this._trash) {
+                this._trash = new Locations.Trash();
+                this._signalsHandler.addWithLabel('show-trash',
+                    [ this._trash,
+                      'changed',
+                      Lang.bind(this, this._queueRedisplay) ]);
+            }
             newApps.push(this._trash.getApp());
+        } else if (this._trash) {
+            this._signalsHandler.removeWithLabel('show-trash');
+            this._trash = null;
         }
 
         // Figure out the actual changes to the list of items; we iterate