Fix resource name conflict compilation issue, following cketti's suggestion.

Use shopping_divider for compiled-in themes, but if it is not found (e.g. previously published themes) check for divider as well.
Switch from FragmentActivity to its subclass ActionBarActivity.

The PopupMenu implementation inside the appcompat library won't work without it -- it depends on some theme attributes that go with Theme.AppCompat.

Now we can start thinking about how to actually use the action bar...