Fix song editing.

Adding new artists and, especially, albums wasn't working.  I was able to add them easily enough via direct inserts into the DB, but thought I'd take a look and see if I could make the UI work.  Here's what I came up with:

Make artist and album drop downs not show blank data and no choices after selection of Add New.
Make text boxes show up after selecting add new.
Add additional text boxes for year and disk number when adding a new album.
Include the disk number of album in drop down selections.