Add Face header support

KMail has support for displaying the Face header, but it does not have support to configure sending it as opposed to the X-Face header, which has a tab in the identity configure dialog. This changes that tab to be able to configure sending both the X-Face and Face header.

This needs support in messagelib (to send the Face header) and kidentitymanagement (to store the Face header): see kidentitymanagement!7 (merged), messagelib!61 (merged).

Here's how it looks: