perf ui browser: Allow specifying message to show when no samples are available to...

perf ui browser: Allow specifying message to show when no samples are available to display

The 'perf top' tool will use that to avoid having a initial blank screen
while collecting the minimum number of samples to sort and display.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lkml.kernel.org/n/tip-89ciceg8cy4442he3t0jzo3f@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>