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
always recompute the dynamic target for a mark

This is a bug fix. Previously, recomputing was done only for refile,
which is wrong: trash target can also be dynamic, and we want to allow
the user to configure more dynamic targets.
documentation fix
mu4e: marks: clarify the way targets are obtained.

:get-target is split into:
- :ask-target (run once per bulk operation)
- :dyn-target (run once per message)

A side benefit is that the existence of the target of a move is not
checked for every message when doing bulk moves.
Merge pull request #524 from jyp/master

Clarify the computation of mark targets
mu4e: support jumping to next, previous unread message

mu4e-headers-next-unread
mu4e-view-headers-next-unread

Use key-bindings ] and [ to go to next, previous unread message.
Merge branch 'master' of github.com:djcb/mu