perf scripts python: exported-sql-viewer.py: Move view creation

perf scripts python: exported-sql-viewer.py: Move view creation

As preparation for adding support for copying to clipboard, create view
in TreeWindowBase instead of derived classes.

Committer testing:

Tested using an old .db used to test some older patches:

  $ python ~acme/libexec/perf-core/scripts/python/exported-sql-viewer.py ~/c/adrian.hunter/simple-retpoline.db

Nothing breaks.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: http://lkml.kernel.org/r/20190503120828.25326-3-adrian.hunter@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>