perf annotate: Cycle thru sorted lines with samples

The annotate TUI now starts centered on the line with most samples, i.e.
the hottest line in the annotated function. Pressing TAB will center on
the second hottest function and so on. Shift+TAB goes in the other
direction.

This way one can more easily sift thru the function hotspots.

Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
LKML-Reference: <new-submission>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>