As documented at
<https://msdn.microsoft.com/en-us/library/office/ff839402.aspx>.

And also extend ov::excel::XInterior with a ThemeColor property, as
documented at
<https://msdn.microsoft.com/en-us/library/office/ff820778.aspx>;
implementation is just a stub for now.

Change-Id: I05f1490cdc366f5db127d340cab5f51efcafa862
Reviewed-on: https://gerrit.libreoffice.org/38522Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>