Converted the API to a gradle project

**Not ready to be merged**

This is the first iteration of changing the API into a separate project.

It will allows us (and others) to reference a concrete API version/build as a dependency instead of importing a specific git commit as git submodule.

Also it will be clearly visible if there is a new API build available. As there can be currently not API changes between AE2 builds, but still have the same version as different builds available.

**Required for the release build**
We need to change our current build definition to create a fatjar/uberjar/shadedjar/[insertNameForMergedJarsHere] including the API. As well as remove the git submodule and add it as dependency as soon as the API is available through a maven repo.