perf scripts python: exported-sql-viewer.py: Add support for multiple sub-windows

perf scripts python: exported-sql-viewer.py: Add support for multiple sub-windows

Use Qt MDI (multiple document interface) to support multiple sub-windows.
Put the data model in a cache so that each sub-window can share the same
data. This allows mutiple views of the call-graph at the same time and
paves the way to add more reports.

Committer testing:

Starts with a "File  Reports  Windows" main menu, from the "Reports" I
can get what was available up to now, the "Context-Sensitivi Call Graph"
option.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: http://lkml.kernel.org/r/20181001062853.28285-14-adrian.hunter@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>