Selection and search should be disabled when there are no photos

Since the visibility of toolbar buttons now depend on the current
PhotosViewModel, we need to reset the toolbar and set the model in a
well defined order. Therefore we reset the toolbar from PhotosEmbed
before setting the model, instead of having it separately listen to
ModeController::window-mode-changed.

https://bugzilla.gnome.org/show_bug.cgi?id=725587