1. Table cell properties were exproted empty if file
     saved as .pptx file.
  2. Now added code to export table cell fill properties
     in 'tcPr'(Table cell properties xml tag)
  3. Added unit test to check exported table cell fill
     properties.

Change-Id: Ica6005a65c7eefb8629c808f2a54764f98badb11
Reviewed-on: https://gerrit.libreoffice.org/14734Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>