Optimize saving to ODF.

This combines the commits ecac0d03d90c and the idea from commit 43b0a39eb91 together to;
1) keep the to-cell anchored shape handling intact and working as expected.
2) improve the saving-speed by some tons of factors by using the RowRepeatStorage to compress identical rows together.

All tested test-documents are still working as expected before vs after the patch. Feedback welcome :)