views: Fix notifications_popup calls on startup

4 views are populated on startup: Albums, Artists, Songs and Playlists. Each populate call goes with a push_loading call to display a loading notification popup and increment a counter. However, push_loading call from PlaylistView never happens because it is placed in _populate method instead of populate. A push_loading call happens in SearchView on startup. It shouldn't be the case as this view is not populated on startup.

The following changes solve this isssue: Moving push_loading call from PlaylistView in the populate method. Moving push_loading call from SearchView in the set_search_text method. PlaylistView does not need a specific _populate method as this is now the same as BaseView.