Between Qt 3 and Qt 4 the behavior of QWidget::polish() changed. What used to happen is
that HistoryPlaylist created the first column (column 0) during construction by putting its
addColumn call in HistoryPlaylist::polish(), which somehow got called before the Playlist
class added its columns.

In Qt 4 the polish method seems to have gone away, so we have to take different measures to
ensure that the HistoryPlaylist gets first dibs on making columns. We do this by altering
the ctor in Playlist to add placeholder columns for subclasses to control later.

This fixes a long-standing JuK bug (since KDE 4.0) that doesn't seem to actually have been
opened in b.k.o.

svn path=/trunk/KDE/kdemultimedia/juk/; revision=1136889