Make Mew Route Group Dialog modal

Prevent interaction with main window while creating a new
route group. This fixes an edge-case: one can currently
delete route that is going to be part of the new group.

- select a single track
- click and drag the left bar to create a group that includes
  the selected track, but don't click "new" yet
- delete the selected track
- click new on the track group window

The group retains a reference to the route.

gtk2_ardour/route_group_dialog.cc

@@ -42,7 +42,7 @@ using namespace std;
 using namespace PBD;
 
 RouteGroupDialog::RouteGroupDialog (RouteGroup* g, bool creating_new)
-	: ArdourDialog (_("Track/bus Group"))
+	: ArdourDialog (_("Track/bus Group"), creating_new)
 	, _group (g)
 	, _initial_name (g->name ())
 	, _active (_("Active"))