explicit direction in PipeWalker

This allows to instanciate the walker as "reversed", i.e.,
makes the list grow from bottom up instead of top down.
smug focus_last in SearchBuffers

In case a search buffer displays more than 200 threads
and one calls `move last`, this will now result in a
new search with reversed search order displayed using a
reversed PipeWalker.

This makes it unnecessary to read all thread id's
when focussing the last element and thus speeds up the UI
considerably.
redraw the UI afer focus [last|first] commands

If one replaces some widget without letting urwid know,
one has to manually trigger it to redraw the screen.
We enforce this to happen after focus_first and focus_last
commands, which might replace widgets in search buffers.

For sake of performance this is *not* done after all other
focus movements.
Merge branch '0.3.5-feature-smug-G'