Emit a single change when changing track selection in the mixer.

Previously changing track-selection emitted 2 "stripable_selection_changed"
events: select (nil); select (new).

This fixes an issue with the FaderPort ctrl surface when editing plugins:
Switching from one track to another follows selection, but if no track is
selected, the FP switches from "Edit Plugins" to "Track" mode.

gtk2_ardour/route_processor_selection.cc

@@ -170,6 +170,7 @@ RouteProcessorSelection::set (AxisView* r)
 	if (!shp.session()) {
 		return;
 	}
+	PresentationInfo::ChangeSuspender cs;
 	shp.session()->selection().clear_stripables ();
 	add (r, true);
 }