filters: MDL-17684 Look for the filter name in filter_myfilter.php first.

filters: MDL-17684 Look for the filter name in filter_myfilter.php first.

This makes filters more plugginable, becuase with this lang file name, get_string will look for the filter name in filter/myfilter/lang/en_utf8/filter_myfilter.php.

To do this, there is a new function filter_get_name in filterlib that contains the logic.

Also, a new function filter_get_all_installed to replace the logic for getting all filters that was duplicated in three places.

filter_get_name no longer does such a nice fall-back if the name is missing, to encourage people to supply the right string. The fallback now looks like '[[filtername]] (filter/tidy)'.