MDL-38147 event-driven cache purging for course categories
MDL-38147 replaced function coursecat::get_all_ids() with coursecat::get_tree()
MDL-38147 coursecat: bug fix in caching contexts
MDL-38147 Added function coursecat::get_children() to retrieve subcategories

Caching results, sorting categories, retrieve only those that are displayed. Added tests
MDL-38147 added function coursecat::search_courses
added function course_get_coursecontacts() temp ???
- MDL-38147 courses in category, back end, unfinished
- MDL-38147 course_search returns instances of course_in_list now
MDL-38147 added event changesincourse