Fix menus as per bug 6499 and 6608.

Fix menus as per bug 6499 and 6608.

* gtkutil.c (xg_event_is_for_menubar): New function (Bug#6499).

* gtkutil.h (xg_event_is_for_menubar): Declare.

* xfns.c (x_menubar_window_to_frame): Take XEvent as second parameter
instead of Window.  Call xg_event_is_for_menubar when
USE_GTK (Bug#6499).

* xmenu.c (x_activate_menubar): Revert previous fix for Bug#6499,
i.e. don't put back ButtonRelease (Bug#6608).

* xterm.c (handle_one_xevent): Pass event to x_menubar_window_to_frame.

* xterm.h (x_menubar_window_to_frame): Second parameter is XEvent*.