emacs: add notmuch-search-edit-search and notmuch-tree-edit-search

...and bind these to "E" in their respective keymaps.

Expected to be called interactively, then using read-from-minibuffer
with current search string as initial contents for editing.
(Noninteractive use makes little sense, but is supported.)

With this one can expand (as an opposite to limit) their
query and have e.g. (some of their) saved searches as search
"templates".

While at it, removed `(defvar notmuch-search-query-string)` from
notmuch-tree.el; it is unused (`notmuch-tree-basic-query` is used
instead).

Thanks to Jose Antonio Ortega Ruiz for his example for notmuch-tree
code, and better interactive use.