Sometimes we want just to print a message on the center of the screen,
like in 'perf top' while we wait for the minimum amount of samples to be
collected before sorting and showing them.

Also expose __ui__info_window() as an optimization for cases where such
message is to be printed while holding the ui lock.

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-uat0f89vfwl2w52kv9wzwd8a@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>