Add support for I18N to Lucid menus.

Add support for I18N to Lucid menus.

* xlwmenuP.h (struct _XlwMenu_part) [HAVE_X_I18N]: Change `font' to be
a fontset.  Add a `font_extents' element.

* xlwmenu.c (xlwMenuResources) [HAVE_X_I18N]: Use a fontset for the
`font' resource.
(string_width) [HAVE_X_I18N]: Use XmbTextExtents;
(MENU_FONT_HEIGHT, MENU_FONT_ASCENT): New macros.
(arrow_width, toggle_button_width, size_menu_item, draw_arrow)
(draw_toggle, draw_radio, display_menu_item): Use them.
(display_menu_item) [HAVE_X_I18N]: Use XmbDrawString.
(make_drawing_gcs) [HAVE_X_I18N]: Don't mess with fonts.
(XlwMenuInitialize) [HAVE_X_I18N]: Initialize font_extents.
(XlwMenuSetValues) [HAVE_X_I18N]: Refresh font_extents if font changes.