app: return visible widgets from a few more gimp_prop() functions

... in particular, this fixes the Rotate Colors prop-gui.

app/propgui/gimppropgui-color-rotate.c

@@ -146,6 +146,8 @@ gimp_prop_angle_range_box_new (GObject     *config,
                     G_CALLBACK (select_all_clicked),
                     dial);
 
+  gtk_widget_show (main_hbox);
+
   return main_hbox;
 }
 

app/widgets/gimppropwidgets.c

@@ -949,6 +949,8 @@ gimp_prop_angle_range_dial_new (GObject     *config,
                           G_BINDING_BIDIRECTIONAL |
                           G_BINDING_SYNC_CREATE);
 
+  gtk_widget_show (dial);
+
   return dial;
 }
 
@@ -1063,6 +1065,8 @@ gimp_prop_range_new (GObject     *config,
   if (sorted)
     gimp_gtk_adjustment_chain (adjustment1, adjustment2);
 
+  gtk_widget_show (vbox);
+
   return vbox;
 }