macosx: create playlist columns at runtime instead of relying on the nib file

macosx: create playlist columns at runtime instead of relying on the nib file

This adds support for the following columns: Track Number, Genre, Album, Description, Date and Language. You can remove and hide them on-the-fly using the Playback menu, since no View menu exists so far.

note that column width and column selection isn't restored on relaunch yet