perf scripts python: exported-sql-viewer.py: Add Time chart by CPU

Add a time chart based on context switch information.

Context switch information was added to the database export fairly
recently, so the chart menu option will only appear if context switch
information is in the database.

Refer to the Exported SQL Viewer Help option for more information about
the chart.
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: http://lore.kernel.org/lkml/20190821083216.1340-7-adrian.hunter@intel.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>