When libtidy removed option 'drop-font-tags' the docs said to use the
'clean' option instead (see MDL-69951). However, 'clean' replaces
legacy style information with classes, the corresponding CSS added in
<head>. With 'show-body-only' (used because we're inserting an HTML
fragment mid-way through <body>) that <head> with its CSS is omitted.
So 'clean' with 'show-body-only' removes legacy style information,
rather than tidying it. Consequently we now drop the 'clean' option.
This means <font> tags are no longer dropped as libtidy currently has
no equivalent to the removed 'drop-font-tags' functionality.
* 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
Leon Stringer