X-Face folding fix and addition of Base64-encoded Face header.

This merge request addresses the long-lasting problem with Pan not folding X-Face strings correctly, as referenced in #131 (closed) and #115.

This works by splitting the X-Face string with spaces every 72 characters so GMIME folds it appropriately.

Face header

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.