WIP: core rewrite mostly PlaylistsView and ArtistsView

PlaylistsView

context menu

• play song
• add song
• remove song

playlist menu

• play playlist
• rename playlist
• delete playlist
• issues when leaving the menu without choosing an action

general and ui

• select first playlist on start
• drag and drop operation
• SongWidget needs to show artist and album
• update playlist button style

PlaylistDialog

• port to new model
• fix critical warnings
• new playlist creation

SmartPlaylists

• favorite playlist is not updated
• are smart playlists updated?

Player

• remove get_mpris_playlist -> use coremodel playlist_sort in MPRIS
• fix play_plause
• remove obsolete code
• stop player if the model is deleted
• update player playlist if the model is updated

ArtistsView

• select first artist on start
• port to ListBox (needs to be fixed)
• strange loading effect when an artist has lot of albums -> fixed
• live updates (depends on check_artist_change) -> FIXME: probably does not work well yet

GrlTrackerSource

• implement check_artist_change

New list style

• AlbumsWidget: mostly done
• ArtistsView: in progress
• PlaylistsView: mostly done
• SongsView: to do

MPRIS

• Playlists use the new api

Misc - cleanup

• DiscSongsFlowBox is not used anymore?
• Rename SidebarRow to PlaylistRow