Style decorations based styleclasses and css

[svenreimers]

For now the decorations are styled using code. This makes reuse of automatic converters, e.g. markdown to RTA complex, because the converter will define all the "styles" to use in code and a user needs to write special code to change them (or a complex API is needed to make this configurable).

So the idea is to use styleclasses instead of the explicit formatting defined in code.