dateMenu: Use icon for message indicator

Using a unicode character here means it may look quite different
from the intended style (for instance with emoji fonts). Avoid
this by providing a custom icon and use that instead.

https://bugzilla.gnome.org/show_bug.cgi?id=766368

js/ui/main.js

@@ -122,6 +122,7 @@ function start() {
     sessionMode.connect('updated', _sessionUpdated);
     Gtk.Settings.get_default().connect('notify::gtk-theme-name',
                                        _loadDefaultStylesheet);
+    Gtk.IconTheme.get_default().add_resource_path('/org/gnome/shell/theme/icons');
     _initializeUI();
 
     shellAccessDialogDBusService = new AccessDialog.AccessDialogDBus();