Make reserved gradle.properties more explicit

[SerVB]

In the discussion, it turns out that there are some Gradle properties that KorGE Gradke plugin uses implicitly.

I haven't been aware of that and made a constant korgeVersion=2.0.0.2 to pass it to the version of plugin, but it was used for version of KorGE lib too, so it crashed the build as KorGE lib versioning differs from KorGE Gradle plugin versioning.

I think this should be made more explicit. So there are two proposals:

1. Document which constants are used by KorGE Gradle plugin.
2. Make KorGE Gradle plugin output to build log if it finds and uses overridden versions instead of predefined ones.