This also has a couple of bugfixes:
- file_info_context_system::get_children() was looking for courses with parent=0, it is impossible, block removed;
- file_info_context_coursecat::get_area_coursecat_description() was checking the wrong capability to view files in description
- file_info class now has new methods get_non_empty_children and count_non_empty_children; added default implementation;
- all core classes extending file_info have their implementation of those methods;
- class repository_local_file used for caching get_children() results is removed;
- class repository_local rewritten to use new methods from file_info;
- added caching of retrieved modules in file_browser::get_file_info_context_module() - this query is slow and executed multiple times on big servers
