Make the formula shape compatible with OOo.

When OOo created a formula, it added an annotation to the mathML.
This was loaded and saved back wrong by KOffice, which made it impossible to load back again.

With the added support for math:annotation, we can now interoperate with OOo fully.
This is despite the fact that OOo saves as an embedded document and KOffice saves inline mathML.

NOTE:
The contents of annotations are not saved back correctly.
This is because the OOo formula editor creates a text version of the formula in the annotation.
If the formula is edited in the formula shape, then the annotation will be wrong.
Better to just save an empty annotation then.