oox: Preserve cell theme color.

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
oox: Preserve table style.

Table style is a property that defines a set of background and line
attributes for cells. These attributes depend on theme settings (e.g.
theme colors).

We added a grabbag to the table object to save the table style name.
We detect that name on export and write it back to the document.

Finally, modified an existing unit test to check this attribute too.

TODO: To get the table style working properly after a roundtrip, we
must preserve it and also check that its values are not being
overwritten by different cell or table properties.

Change-Id: Id0e022a389561960c21ab874db33649499735024