tools/kvm_stat: reduce perceived idle time on filter updates

Whenever a user adds a filter, we
* redraw the header immediately for a snappy response
* print a message indicating to the user that we're busy while the
  noticeable delay induced by updating all of the stats objects takes place
* update the statistics ASAP (i.e. after 0.25s instead of 3s) to be
  consistent with behavior on startup
To do so, we split the Tui's refresh() method to allow for drawing header
and stats separately, and trigger a header refresh whenever we are about
to do something that takes a while - like updating filters.

Signed-off-by: Stefan Raspl <raspl@linux.vnet.ibm.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>