widgets: Rename variable playlist to playlists
widgets: Add PlaylistWidget
view: Use PlaylistWidget in Playlists view
window: Properly get the model for Playlists view
query: Add queries for searching with matching playlist name

This also updates queries for any field matches to check if the search
text is a playlist name.
searchbar: Add Playlist to Match options
view: Add Playlist to Match options
query: Add queries for searching Playlists
view: Show playlists in search results
view: Fix selection on playlists in search results
window: Remove playlist too in Search view
view: Close Search view when showing playlist and it was deleted
query: Allow changing the solution sequence modifiers
query: Sort songs by title in search results
widgets: Rename add_playlist_iter to addPlaylistIter
widgets: Select playlist if already exists in Add to Playlist dialog

https://bugzilla.gnome.org/show_bug.cgi?id=728331
widgets: Use list box in Add to Playlist Dialog
view: Don't add new playlist if Playlists view is not yet initialized

This will prevent new playlists to be shown multiple times if they are
created before the Playlists view is shown.
widgets: Add the playlist song to the correct model

This ensures that if we move to a another playlist while the previous
playlist is still loading, the songs won't get mixed up with the songs
of the new playlist.
widgets: Update the songs count only if song added to current playlist