perf scripts python: exported-sql-viewer.py: Add help window

Add a window to display help. It is also possible to display the help
only, by using the option "--help-only" instead of a database name.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: http://lkml.kernel.org/r/20181104151238.15947-4-adrian.hunter@intel.com


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