perf ui browser: Add 's' key to filter by symbol name

Now user can enter symbol name interested via ui_browser__input_window,
and perf can process it using hists__filter_by_symbol(). Giving empty
symbol (by pressing 's' followed by ENTER) will disable the filtering.

Cc: Ingo Molnar <mingo@elte.hu>
Cc: Namhyung Kim <namhyung@gmail.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1331887855-874-3-git-send-email-namhyung.kim@lge.com
Signed-off-by: Namhyung Kim <namhyung.kim@lge.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>