The main impact of this change is to introduce a new abstraction
above RBSource that allows the inclusion of things that don't
contain music and cannot be played from.  For want of a meaningful
name, the new class is called RBDisplayPage.  A few of the basic
methods from RBSource are now on the RBDisplayPage class, but
otherwise this doesn't change any existing sources.  The DACP
pairing source and the last.fm profile source are now direct
subclasses of RBDisplayPage.

RBDisplayPageModel and RBDisplayPageTree are reimplementations of
RBSourcelistModel and RBSourceList respectively, with a few minor
bugs fixed and with the addition that the page group expansion state
is now stored in gconf.

RBDisplayPageGroup replaces RBSourceGroup, and becomes an actual
object that exists in the page tree.  It's also a subclass of
RBDisplayPage, but page groups cannot be selected in the page tree
and don't have any visible content.  The same predefined groups
exist.  Rather than being specified as a property on an RBSource
instance, the group for a page is determined by where it is inserted
into the page tree.