perf scripts python: exported-sql-viewer.py: Factor out ReportVars

Factor out ReportVars to provide a single container for information from
report dialogs.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>