mu4e: add mu4e-headers-find-if-next, mu4e-headers-next-unread

In the headers-view, allow for movig to next and previous
unread/untrashed messages using tab/backtab.

Built on top of the convenience function mu4e-headers-find-if-next.

Update docstrings.
Put the definitions of marks in a single structure

This commit puts all what defines a mark (how to display, how to handle
targets, what action to apply, ...) in a single entry of a list. The list
is stored in a variable.
This should allow for customization of marks.
Merge pull request #523 from jyp/master

mu4e: put the definitions of marks in a single structure
configure.ac: bump version to 0.9.11
mu4e: update documentation
Merge remote-tracking branch 'refs/remotes/upstream/master'