Publish Gradle plugin marker

[Goooler]

Support applying Gradle plugin like:

plugins {
  id 'com.mobidevelop.robovm' version '2.3.20'
}

See https://docs.gradle.org/current/userguide/plugins.html#sec:plugin_markers.

[dkimitsa]

hi,
it's nice to have.
however it will break existing project compatibility as artefact will have different id.

[Goooler]

We can note this breaking change in the changelog, for ones who are upgrading from older versions to know.

[Goooler]

It would be nice to support this feature to follow Gradle practice.

[Goooler]

Fixing this in #766.