C# interop: mark all optional parameter as optional and make style param conversion functions static methods

[kMutagene]

1. All optional parameters should get the optional tag used in the Chart type (see Axis.init all arguments are mandatory in C# #133)
2. Style param convert functions are FSharpFunc instead of invokable methods -> add members/override: ToString() and Convert()

[kMutagene]

should be fixed with the next release