Typesafe filling mode

[Joelius300]

Describe the feature request

Use the filling modes for Line and Radar chart in a typesafe manner.

The properties are already there in LineDataset and in RadarDataset.

• In LineDataset it's of type bool so you can only disable it or use 'origin' (alias when true).
• In RadarDataset it's of type object and there is a description to what values you can use.

Docs

Describe the solution you'd like

We need a class FillingMode (derives from ObjectEnum) where you can use either an int, a string or a bool (private constructors). The supported values are here and you can also see the use cases below.

Here are the use cases we need to support:

• Fill = FillingMode.Relative(2)
• Fill = FillingMode.Relative(-1)
• Fill = FillingMode.Absolute(1)
• Fill = FillingMode.Disabled -> We might also use FillingMode.Off
• Fill = FillingMode.Origin
• Fill = FillingMode.Start
• Fill = FillingMode.End

A nice to have would be to allow implicit boolean conversion which would be defined like this:

• false = FillingMode.Disabled
• true = FillingMode.Origin

[Joelius300]

Should be done as part of #96.