This adds 'Format as' submenu to the preview pane context menu.
The submenu is available only for text/plain parts or parts
with a source code (we support about 40 various types). Using
the 'highlight' utility, the formatter processes the part and
highlights the source code it contains.
(discussion in bug #680026)
audio-inline, itip-formatter, prefer-plain, tnef-attachments
and vcard-inline plugins were converted to modules so that they
can fit into concept of the new formatter.
Every module still installs .eplug file, because there is no
suitable API at the moment to register plugins to the plugins dialog
and to extend the Preferences dialog.
