[DRAFT] convenience class for dealing with ODF styling information.

The class parses the styling information and allows accessing it like e.g.
 fontWeight = style.getTextStyle("T1").text.fontWeight;
If T1 does not define, fontWeight is retrieved from the parent style and
eventually the document default and the implementation default.
This is achieved by setting the object prototype to the style parent.
Parsing of styles is done lazily (when getStyle()) is called.
When the styling information, changes, this class needs to be invalidated, so the information is parsed again when accessed.