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