- added class format_base as the base for all course formats
- added class format_site for the pseudo-format 'site' used for displaying activities on the front page
- added class format_legacy that overrides format_base functions with calling old-style 'callback_xxx' functions
- replaced all usage of 'callback_xxx' functions with format_base methods
- made arguments of get_section_name(), get_course_url() more flexible
- deprecated function get_generic_section_name(), it's contents is moved to format_base
- global_navigation::format_display_course_content() is removed, plugins can supress the sections navigations using extend_course_navigation()