Gradelize Build

[MeFisto94]

In order to move the build to gradle, a few things have to be done, with the most tricky part being the support of jstackalloc, which has a custom ant task we still need to execute (or rewrite for gradle).

In general, I think we should make jbullet.jar contain all it's needed libs like jstackalloc, vectormath (which has a java3d:vectormath maven coordinate) and ant for the custom task.
The alternative would be hosting yet another maven addressable version of jstackalloc which works with java8+, which is something I would like to avoid.

Thus I could imagine those dependencies being built as subprojects and then the ant tasks being filtered out again.
Also our old distribution didn't include the demos of jbullet, we should thus also filter them out of the creation.

Lastly we can/should add CI Support. Travis or Github Actions.