perf report: Support multiple events on the TUI

The hists__tty_browse_tree function was created with the loop to print
all events, and its equivalent, hists__tui_browse_tree, was created in a
similar fashion, where it is possible to switch among the multiple
events, if present, using TAB to go the next event, and shift+TAB
(UNTAB) to go to the previous.

The report TUI now shows as the window title the name of the event and a
leak was fixed wrt pstacks.

Cc: Frédéric Weisbecker <fweisbec@gmail.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Stephane Eranian <eranian@google.com>
Cc: Tom Zanussi <tzanussi@gmail.com>
LKML-Reference: <new-submission>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>