perf hists browser: Add support for showing columns header

perf hists browser: Add support for showing columns header

Open up space to show a one-line header text whenever 'H' is pressed,
hide it on another key press.

Follow up patch will format this line from the set of headers used.

Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/n/tip-m894d6qk30h3qofw4k8neq4q@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>