When some background color is set to a table cell, it is stored in the
cell properties tag <tcPr> like this:

  <w:shd w:fill="ECD2B6" w:color="auto"
         w:themeFill="accent6" w:themeFillTint="66" w:val="clear"/>

The theme-related attributes in w:shd were not being preserved. To
fix this I added an InteropGrabBag to the cell properties object,
which is filled with the attributes of w:shd to be checked on export.

The exporter checks if the cell color is still the original color that
was imported from the file, if it is not it means the user has
manually changed it during the edition and the new color is written
instead.

Finally, added a unit test for theme attributes on tables.

Change-Id: Ica8091b5eb4075e51912a255650a1d9d64f5767a