in a new class nsMarkupDocument. The point of nsMarkupDocument is
to hold methods which are common to HTML and XML but not relevant
to the base nsDocument class.
2) Added Selection support methods for Rod.
3) Added LookupProperty to nsCSSProperty:
Given a property ID and Property Index returns back the string representation
of the property. This was need to convert CSSValues back to strings for saving.
TODO: Make sure that our URL Information stored in a CSSDeclaration contains
enough information to return it to it's original form on output.
