PlaylistDialog::getInstance was duplicating the playlist widget
instanciated in main interface. This explains the empty playlist
bug of the popup dialogs replacing the current playlist by
parenting their menu using getInstance().
To avoid further mistakes, PlaylistWiget's constructor is now only
available to playlistdialog, which also manages the widget
through the different configurations (embedded or floating).
Dialog also now saves and restores geometry, which only happened
when the widget was on its side.