Fixed classpath resolution for IPA creation & Tycho update

When exporting an IPA via the plugin, the classpath resolution of the selected Java project(s) was incomplete. It appears that `JavaRuntime#computeDefaultRuntimeClassPath` does not resolve the dependencies of projects the iOS project depends on, e.g. https://github.com/libgdx/libgdx/issues/1877

I fixed this by recursively resolving the classpaths of all involved projects.

Additionally i updated Tycho to version 0.20.0. Version 0.16.0 is incompatible with Maven versions > 3.0.5. I can split up the PR if you want.