X-Face folding fix and addition of Base64-encoded Face header.
This works by splitting the X-Face string with spaces every 72 characters so GMIME folds it appropriately.
This merge request also adds a new "Face" setting in the Posting Preferences that allow the user to specify a 998-byte Base64 string that encodes a 48x48 PNG file, as described in this document (archive). Pan is already able to display them.
Should it be unwanted, it may be easily reverted. Changes regarding the Face and X-Face headers are completely independant.