emacs: notmuch-tree-outline-mode

With this mode, one can fold trees in the notmuch-tree buffer as if
they were outlines, using all the commands provided by
outline-minor-mode.  We also define a couple of movement commands
that, optional, will ensure that only the thread around point is
unfolded.

The implementation is based on registering a :level property in the
messages p-list, that is then used by outline-minor-mode to to
recognise headers.

Amended by db: Copy docstring to manual and edit for presentation. Add
two tests. Fix typo "wether".