Add "Use Global" option for "Show Page Heading" setting of menu item

This is an attempt to address Issue #4168.
The modifications allow show_page_heading for a menu item to be set to JGLOBAL_USE_GLOBAL, JYES or JNO. When it is set to JGLOBAL_USE_GLOBAL, com_menu's global show_page_heading setting is used when display the page.