#131 PlaylistSelector should not enable buttons

[Sean Starkey]

The PlaylistSelector has some buttons that should be
disabled until the user takes some action.

For example, the "Add Tracks To Playlist" button should
not be enabled until a playlist is selected.

In the add files to playlist dialog, the "Create New
Playlist" should not be enabled until files are added.

[Carnell Washington Jr]

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Marc let me know if you need help with this. I just
implemented it for the playlist editor so its fresh in my head.

[Marc A. Brown]

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If you want to work on it, go ahead. I've had zero time for
working on it (darned 24 hour day - why can't it be
longer?), so *someone* may as well get it fixed.