perf ui browser header: Provide feedback on unhandled hotkeys

Don't just eat unknown keys without providing visual feedback and
instructions on how to see which ones are assigned.

Suggested-by: Ingo Molnar <mingo@kernel.org>
Tested-by: Ingo Molnar <mingo@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: James Clark <james.clark@linaro.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/Z_TYux5fUg2pW-pF@gmail.com


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