Matt, I have opened this as a subtask for bug 10889.

I assume that we will implement this as a server (SC) preference.

Weldon, we need UI design for each of the UIs to cover the cases when:
(a) one tries to add one or more tracks to a playlist that is already full (at
maximum length);
(b) one tries to add several tracks to a playlist that is almost full, such
that not all the tracks can be added.

Also, we need to consider the semantics that we want when attempting to add a
track or tracks that would not fit but which could be made to fit by removing
already-played tracks from the start of the playlist.