window: Set default icon

Set default icon instead of just window icon, so it gets picked up by
pulseaudio as application property. Fixes the incorrect icon showing in
gnome-control-center.

https://bugzilla.gnome.org/show_bug.cgi?id=709073

gnomemusic/window.py

@@ -77,7 +77,7 @@ class Window(Gtk.ApplicationWindow):
         selectNone.connect('activate', self._on_select_none)
         self.add_action(selectNone)
         self.set_size_request(200, 100)
-        self.set_icon_name('gnome-music')
+        self.set_default_icon_name('gnome-music')
         self.notification_handler = None
         self._loading_counter = 0