Searching with a window over notmuch vfolders

Searching with a window over notmuch vfolders

When searching over a very large result, the result will be very slow for mutt
to load. So, inspired by sup's feature to iteratively load the results, I figured
adding a couple of commands to move an user set window backward/forward in time
could be useful. And a new command extending the default vfolder-from-query function
is being exposed.

The current window information is user settable, so the user can make a macro that
would do useful stuff about a search (like moving the window's position around, or
further tweaking current search).

* addition of four configuration variables, the first two are aimed at configuring
    the behaviour of the feature, the last two are exposing internals to make it
    hacking friendly:
  - nm_query_window_duration: duration for each window
  - nm_query_window_timebase: time base for each window
  - nm_query_window_current_search: the search being displayed as vfolder
  - nm_query_window_current_position: the current window's position
* addition of three bindable commands:
  - windowed-vfolder-from-query: execute a search with a window applied
  - windowed-vfolder-backward: move the window backwards in time
  - windowed-vfolder-forward: move the window forwards in time

Closes: #278