fix a likely logic error in GtkMenu handling of enter events

On macOS popping up a menu with a position function that leads the menu to be
under the mouse pointer generates the same initial set of enter events as on
linux, but this is then followed  by some leave events and then a repeat of the
same set of enter events.

this exposes what appears to be a logic error in gtk_menu_enter_notify().

Nonlinear enter events (i.e. where the menu pops up under the mouse, so the
mouse was never actually moved into the menu's window) should not cause a reset
of priv->seen_item_enter. This value, when true, is used to force the next
received button release event to be handle as an menu activation event. This
should only happen when the mouse has actually been moved by the user into the
menu (as already indicated in existing comments). The value has this effect by,
on the next enter event for a menu item, if already set, forcing
menu->activate_time to zero, which in turn will cause menu activation on button
up/release.

This bug doesn't appear on Linux (or Windows) because the event sequence
associated with a menu popup is different. The lack of another set of enter
events means that menu->activate_time is never reset, and so the behavior is as
expected.

This change makes the setting of priv->seen_item_enter be idempotent with
respect to non-linear enter events, which I believe is the intended behavior.

libs/tk/ytk/gtkmenu.c

@@ -3906,9 +3906,8 @@ gtk_menu_enter_notify (GtkWidget        *widget,
 		   */
 		  menu_shell->activate_time = 0;
 		}
+	      priv->seen_item_enter = TRUE;
 	    }
-	    
-	  priv->seen_item_enter = TRUE;
 	}
     }