filtering related improvements 2.5 META

List of filtering related issues targeted for 2.5, there are multiple known problems in the design of filtering subsystem such as:

1/ legacy filters in activities - this complicates the code and testing significantly, it is also one of the last places that did not use frankenstyle
2/ restrictions caused by text caching - thanks to the obsoleted caching table filters must not use PAGE, $USER, javascript and other globals; solution is to move the caching from core into filters
3/ we can use MUC in multiple filter related areas