Restore selectable flag while painting a menu item.

In `menuitem_paint` when a function is not allowed such as Resize, the selectable flag is cleared. However it is never restored. Since the Resize menu item is being reused, it disables selecting Resize on all other windows. This commit re-enables the flag.

fvwm/menuitem.c

@@ -461,6 +461,7 @@ void menuitem_paint(
 	}
 	else
 	{
+		MI_IS_SELECTABLE(mi) = True;
 		gcs = ST_MENU_INACTIVE_GCS(ms);
 		off_gcs = ST_MENU_INACTIVE_GCS(ms);
 	}