perf ui/gtk: Support flat callchains

The flat callchain mode is to print all chains in a simple flat
hierarchy so make it easy to see.

Currently perf report --gtk doesn't show flat callchains properly.  With
flat callchains, only leaf nodes are added to the final rbtree so it
should show entries in parent nodes.  To do that, add parent_val list to
struct callchain_node and show them along with the (normal) val list.

See the previous commit on TUI support for more information.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Tested-by: Brendan Gregg <brendan.d.gregg@gmail.com>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Kan Liang <kan.liang@intel.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1447047946-1691-10-git-send-email-namhyung@kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>