Include Oxyplot as a submodule instead of NuGet package

[Happypig375]

https://github.com/oxyplot/oxyplot-xamarin/issues/100#issuecomment-572769837

I recommend including the OxyPlot Xamarin as a git submodule/subtree and building these projects yourself from the latest source code.

[TimLariviere]

Sorry, but I don't think we should take the responsibility to build it ourselves. Fabulous.XF extensions are meant only as wrappers around external packages maintained by others. We already have enough to do with Fabulous alone.

[Happypig375]

external packages maintained by others

But the package is currently unmaintained which is over 3 years old now. The submodule approach is the maintained alternative.

[TimLariviere]

As per https://github.com/oxyplot/oxyplot-xamarin/issues/84, I only really see it as a way to offload the maintenance to whoever uses the package because they don't have the time/people willing to maintain it properly and make sure it works on all platforms. Which is fine.

But if we were to build and release an OxyPlot.XF package, we would take all the responsibilities:

• Set up all the build and release processes
• Make sure it works on all platforms
• Fix bugs and update it every now and then
• Also people would come here to file issues and features for OxyPlot.XF

And those aren't the goals of Fabulous.

We only aim to provide a compatible wrapper to make OxyPlot usable in Fabulous.

If the wrapped package ends up unmaintained, people are free to participate to the open-source repository of OxyPlot and release a new version there. Fabulous won't do it for them.