* fix unit test after thread-sorting change (fixes #413)
* don't mark the mail-header-separator as read-only

  this seems to cause problems in some cases, where message-mode
  apparently requires the text to be writable.
* make mu4e-view-url-regexp a public variable

  thus, users can more easily experiment with this
* mu4e: add fallback for `delete-all-overlays'