Box2d chain support

[B4rtware]

Adds simple chain support for box2d.
Adds a simple test demo. I don't know much about chains but for my current use case I didn't encounter any issues.
Should the demo be hidden?

[fabmax]

Nice work! I added the missing Android bindings and changed the vec2 array creation to use a single b2vec2 instead of creating a new one per array entry. I think the demo can be hidden. Otherwise I would like it to be a bit more interactive 😄

[B4rtware]

Thank you, I was not able to get the android module recognized by gradle/intellij. Thats why it built without any errors on my end.

The demo is now hidden. Since it will be used to sandbox my game, I will try to update the demo accordingly, maybe a 2d character controller, ladders, portals ...

[fabmax]

For android there's a gradle task to enable it: :enableAndroidPlatform, or in IntelliJ's gradle panel under Tasks -> build setup -> enableAndroidPlatform. It's disabled by default to lower build times and so that you don't need an Android SDK for building kool.