Fix broken scroll wheel in radio and rotator controllers

Fixes #362.
Fixes #351.

Author: csete

src/gtk-freq-knob.c

@@ -358,6 +358,7 @@ GtkWidget      *gtk_freq_knob_new(gdouble val, gboolean buttons)
             g_signal_connect(knob->evtbox[i],
                              "button_press_event", (GCallback) on_button_press,
                              widget);
+            gtk_widget_add_events(GTK_WIDGET(knob->evtbox[i]), GDK_SCROLL_MASK);
             g_signal_connect(knob->evtbox[i], "scroll_event",
                              (GCallback) on_button_scroll, widget);
 

src/gtk-rot-knob.c

@@ -528,6 +528,7 @@ GtkWidget      *gtk_rot_knob_new(gdouble min, gdouble max, gdouble val)
 
         g_signal_connect(knob->evtbox[i], "button_press_event",
                          (GCallback) on_button_press, widget);
+        gtk_widget_add_events(GTK_WIDGET(knob->evtbox[i]), GDK_SCROLL_MASK);
         g_signal_connect(knob->evtbox[i], "scroll_event",
                          (GCallback) on_button_scroll, widget);