Updating the DB is buggy

[krolyxon]

when you do database update with by pressing 'U', it updates the data base and if some new songs are added, it shows their entry as "Not a Song".

this happens because the song_filenames list is immidiately updated, even when the database update is ongoing.
fix this by using a checker member to see if the status.updating_db is None, and if its none update it.