Add different face attribute for the mode-line

There is an additional face attribute for the mode-line.
Update mu4e-headers.el
Update mu4e-vars.el
Merge pull request #478 from odi/master

mu4e: Add different face attribute for the mode-line
mu4e: Use save-excursion to leave point before signature

Based on the patch by janisteunissen.
  https://github.com/jannisteunissen/mu/commit/7f8b598e009c36ad0d6d2808c7a83341f0051f8f
Fix encrypted multiparts and attachments indexing

Pull request #483 does not handle encrypted multiparts properly.  It
used to just verify the signature and not process the parts of the
multipart.  This commit resolves this issue.

Additionally it did not index attachments properly and in the case of a
multipart directly containing more than one multiparts resulted on non
unique indexing of attachments/parts.  This commit resolves this issue
as well.
threader: try to handle pathless messages
Remove left over argument from merge
Merge pull request #487 from zakkak/master

Fix encrypted multiparts and attachments indexing