perf annotate browser: Allow toggling the visualization of source code lines

Just press 'S' on any assembly line and the source code will be hidden
while the current line remains selected. Press 'S' again to show them
back.

Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-efmxm5etouebb7es0kkyqqwa@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>