Emit signal when dropdown changes via scroll-wheel

This fixes Virtual-keyboard velocity selection when scrolling
to numbers that are not explicitly in the menu-list.

(hopefully this does not introduce some recursive signals
for other ArdourDropdowns, fingers crossed).

libs/widgets/ardour_dropdown.cc

@@ -99,6 +99,7 @@ ArdourDropdown::set_active (std::string const& text)
 		}
 	}
 	set_text (text);
+	StateChanged (); /* EMIT SIGNAL */
 }
 
 bool