build: move libbanshee inside the Banshee.GStreamer backend folder

In the same way libbnpx11 unmanaged lib is inside Banshee.NowPlaying.X11
folder (the extension that depends on it), libbanshee is now being moved
to the folder of the backend extension Banshee.GStreamer; which is
currently the default, but may be just optional soonish (so we can
make this conditional compilation easier too).