Right now, SourceMarkers are implemented with TextMarks as a backend.
However this does not fit the model that most people use the `SourceMark'
for. Since the mark displays an icon on the line which it is places, it is
natural to limit 1 marker per type per line.

Also, the functions should be reworked to accomodate this. For example,
right now in MD we have:

		public void NextBookmark ()
		{
			TextIter loc = GetIterAtMark (InsertMark);
			int ln = loc.Line;
			
			IntPtr nextMarker = gtk_source_buffer_get_next_marker (Handle, ref loc);
			IntPtr firstMarker = nextMarker;
			bool first = true;
			while (true) {
				// Thats a wrap!
				if (nextMarker == IntPtr.Zero)
					nextMarker = gtk_source_buffer_get_first_marker (Handle);
				
				IntPtr nm = gtk_source_marker_get_marker_type (nextMarker);
				string name = GLibSharp.Marshaller.PtrToStringGFree (nm);
				if (name == "SourceEditorBookmark") {
					gtk_source_buffer_get_iter_at_marker (Handle, ref loc, nextMarker);
					
					if (! first || loc.Line != ln)
						break;
				}
				
				nextMarker = gtk_source_marker_next (nextMarker);
				
				first = false;
			}
			
			PlaceCursor (loc);
		}

Which will place the bookmark on the next line (yes, that is a wierd combo
of bindings + function invokes, this is because of refcounting issues +
gtype issues)

Really this function should be built in, so that you can just `go to the
next marker of this type'