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())

The media options dialog has currently an open bug regarding the
behavior of the nolink option. Additionally will wrong syntax be
inserted when using Internet Explorer < 8. Since this can't be
fixed on time, this patch disables the media options for the release.

de / typo fix

SafeFN encoding now always end the encoding block by appending a dot at the end of the file name. This is necessary since the file name may get an extension which is not encoded.