rename mu4e-maildirs-extension-header-face to mu4e-maildirs-extension-maildir-face

Signed-off-by: Niv Sardi <xaiki@evilgiggle.com>
go back to propertizing the unread part of the maildir line

Signed-off-by: Niv Sardi <xaiki@evilgiggle.com>