perf ui browser: Add a warn on unhandled hotkey helper

perf ui browser: Add a warn on unhandled hotkey helper

Will be used by the various browsers, looks like:

                                        ┌─Warning!───────────────────────────────────────────┐
  29.15 │ 3c:  cmpl    $0x3,(%rbx)      │                                                    │
   3.77 │    ↓ je      51               │'Ctrl+V' key not associated, use 'h' to see actions!│
  16.59 │      movq    0x3b0(%rbx),%rdx │                                                    │
  30.65 │      movl    0x8(%rdx),%eax   │                                                    │
   3.77 │      cmpl    %eax,%edi        │Press any key...                                    │
   0.25 │    ↓ jb      82               └────────────────────────────────────────────────────┘

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>