adding RecentsMRL::toPlaylist()
refactoring functions to eliminate repeating code
refactoring public DialogProvider::saveAPlaylist() to private DialogProvider::saveAPlaylist(playlist_t *p_playlist, playlist_item_t *p_node)
and making new DialogProvider::savePlayingToPlaylist() and DialogProvider::saveRecentsToPlaylist()

this commit includes fixes related to Jean-Baptiste Kempf code review.
 - fixing whitespace problem
 - RecentsMRL::toPlaylist null handling
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>