filters: MDL-7336 change weblib to use the new code

filters: MDL-7336 change weblib to use the new code

* Rename filter base class from filter_base to moodle_text_filter
* Remove unnecessary explicit constructors in moodle_text_filter
subclasses
* New filter_manager class, rather than static methods in filter_base
* Move some logic out of weblib, and into filter_manager
* Count filtering ops when $CFG->perfdebug on, via
performance_measuring_filter_manager
* Kill unused filter_string function. Petr said it should have been
private to weblib