Added new virtual methods to Playlist -- readOnly() and columnOffset().

columnOffset() indicates the position at which the Playlist class can expect
to find the standard set of columns.  This let me add a "time" column to
the history playlist.

I adjusted the code in other places to use these two new methods.

Also allow the history playlist to have multiple copies of the same item;
this is still buggy at the moment since this API feature hadn't yet been
used.

svn path=/trunk/kdemultimedia/juk/; revision=241800