Support the new batched query results from the mu server; these are much faster
to render (2-3x it seems).
Rearrange the code a bit to avoid byte-compiler warnings.
Obsolete mu4e-header-func, to be replaced by mu4e-headers-append-func.
Replace obsolete variable alias with obsolete function alias for mu4e-headers-search-edit.
Update docstring for mu4e-view-search-edit to reference the new function.
Split off the search functionality from mu4e-headers.el into a new
mu4e-search.el.
Clean up things a bit and create a minor mode in which to add the keybindings.
Enable this in main/headers/view.