Add a way to turn off toggle spacing in menus

Reserving toggle space is nice for consistency in main menus
and context menus, but it gets in the way in special situations,
such as combo boxes, tabular menus, etc.

docs/reference/gtk/gtk-sections.txt

@@ -2420,6 +2420,8 @@ gtk_menu_get_title
 gtk_menu_set_monitor
 gtk_menu_get_monitor
 gtk_menu_get_tearoff_state
+gtk_menu_set_reserve_toggle_size
+get_menu_get_reserve_toggle_size
 <SUBSECTION>
 gtk_menu_popdown
 gtk_menu_reposition