Keep current message selected when clearing or changing filters and when
toggling threads.
Add -s flag to the clear command to also clear the selected message and
set cursor to the top of the message list.
Implements: https://todo.sr.ht/~rjarry/aerc/36
Signed-off-by: Koni Marti <koni.marti@gmail.com>
Acked-by: Robin Jarry <robin@jarry.cc>