perf hists browser: Add 'm' key for context menu display

With horizontal scrolling, the left/right arrow keys are used to scroll
columns and ENTER/ESC keys are used to enter/exit menu.  However if
callchain is recorded, the ENTER key is used to toggle callchain
expansion so there's no way to display menu.  Use 'm' key to display the
menu for this case.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1444694521-8136-1-git-send-email-namhyung@kernel.org


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