perf scripts python: exported-sql-viewer.py: Add ability to shrink / enlarge font

Shrinking the font allows more information to display.

Committer testing:

Works, tested with the convenient Control+Shift+'+' and Control+'-' as
well with the more cumbersome top menu "Edit" + "Enlarge/Shrink font"
options.

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/20181001062853.28285-16-adrian.hunter@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>