Added support for combo charts (ie. bar + line chart)

[dvdkroft]

BarChart widget updated to support multiple chart types in one chart.

[JelteMX]

• I get that you can use an Enum as Attribute type, but... in the code you do check which type it is. So right now it doesn't check if it's bar, line or banana. Which means that the type could be set to a nonsensical type. (We cannot enforce the type that should be in an Enum)
• It should also be documented in the documentation
• This is now only implemented in the BarChart, but what about the LineChart?

I think it's best to create a separate chart type. Something like MultiChart, that implements this. (BarChart and LineChart share a huge portion of the code, so it should be possible to create a 'hybrid' type like you want to do here)

I'll note your PR and see if we can create a different type of chart.