Ignore units on FontSize in shapeMarshall

This fixes an issue reported by @aleksandra-tarkowska where text label ROIs break ROI display in web (due to font size units). https://github.com/openmicroscopy/openmicroscopy/pull/3245#issuecomment-65805182

To test, add a text label Shape to an image in Insight, then test that it displays in web.
@will-moore perhaps setting

```
shape.getFontSize() is not None and shape.getFontSize().getValue() or 12
```
could be enought