perf gtk/hists: Use GtkTreeStore instead of GtkListStore

perf gtk/hists: Use GtkTreeStore instead of GtkListStore

The GtkTreeStore can save items in a tree-like way.  This is a
preparation for supporting callgraphs in the hist browser.

Reviewed-by: Pekka Enberg <penberg@kernel.org>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1370337737-30812-2-git-send-email-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>