perf report tui: Improve multi event session support

When multiple events were used in 'perf record', allow the user to
choose which one is wanted before showing the per event histograms.

Annotations will be performed on the chosen event.

Allow going back and forth from event to event quickly using just the
arrow keys and enter.

Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Tom Zanussi <tzanussi@gmail.com>
Cc: William Cohen <wcohen@redhat.com>
LKML-Reference: <new-submission>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>