perf, sched migration: Make it vertically scrollable

With scheduler traces covering more than two cpus, rectangles
of the CPUs 3 and more are not visibles.

This makes the vertical navigation scrollable so that all of the
CPUs rectangles are available.

We also want to be able to zoom vertically, so that we can fit at
best the screen with CPU rectangles, but that's for later.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Nikhil Rao <ncrao@google.com>
Cc: Tom Zanussi <tzanussi@gmail.com>