playlistdialog: Adwaitaize

Make PlaylistDialog use Adwaita 1.4 widgetry and drop Gtk.Dialog use in the process.

Related to #579 (closed)

progress (17 jan)

The UI bits are lookin ok.

The back of it is not hooked up. It can be shown by using albumwidget and then the album menu button's 'add to playlist'.

21 jan

All bits working, depends on !1034 (merged)