one off, special case rec-en button for now. (circle may need tweaking)

In the long run we want a class hierarchy:
CairoWidget
-> ArdourWidget
-> ArdourLabel
-> ArdourButton
-> SpecialArdourButton(s)

gtk2_ardour/ardour_button.h

@@ -40,6 +40,7 @@ class ArdourButton : public CairoWidget , public Gtkmm2ext::Activatable
 		unused = 0x10,
 		Menu = 0x20,
 		Inactive = 0x40, // no _action is defined AND state is not used
+		RecButton = 0x80, // tenative, see commit message
 	};
 
 	static Element default_elements;