* library/managedwin.ith (_geometry): Use a protected variable

	* library/managedwin.ith (_geometry): Use a protected variable
	instead of a global from ManagedWin::reveal.
	(open): Make arg list reflect managedwin.itb.
	* library/managedwin.itb (window_name): Use class variable
	_top instead of recomputing the toplevel every time.
	(reveal): Ditto.
	Use class variable "_geometry" instead of global variable.
	(shutdown): Do not save ModalDialogs.
	(_create): If centering the new window, use libgui's
	center_window to do it.