emacs/tree: enable moving to next thread in search results
This introduces a new function called
notmuch-tree-next-thread-from-search which is analogous to
notmuch-show-next-thread. It will switch to the next or previous
thread from the parent search results.
We rename notmuch-tree-{prev,next}-thread to a more descriptive
notmuch-tree-{prev,next}-thread-in-tree to reflect the fact that it
only moves to the next thread in the current tree.
notmuch-tree-next-thread now switches to the next thread in the
current tree first, but if there are none, it looks for the next tree
in the search results.
This makes notmuch-tree feel more like notmuch-show when using the
M-Enter, M-n and M-p bindings.