This patch only adds the capability to the Difference Engine lib, not to
the user interface, yet.

This patch does two things: It makes sure the native json_decode() will
honor the JSON_LOOSETYPE option of the class and it also adds way
to skip the use of the native function completely. The latter is
necessary for slightly non-standard JSON data.

p_purge_metadata now updates the metadata cache and the INFO array like the other metadata writing functions

this is mainly for use with plugin:upgrade and thus will be
cherrypicked into the current stable branch as well.

make custom buttons possible with html_btn() without the need of global $lang (more consistent with tpl_pagelink())