Maven central requirements

According to maven central requirements, these are needed:

• JavaDoc, QTTA-91
• File Checksums DONE
• GPG Singing, Need to add
  • https://docs.gradle.org/current/userguide/publishing_signing.html#publishing_maven:signing   
  • https://central.sonatype.org/publish/requirements/gpg/{}
• Sufficient Metadata
  • Correct Coordinates DONE{}
  • Project Name DONE
  • Description
  • URL DONE

Because QtGP uses java-gradle-plugin, can't declare these in publications normally:

• License Information, Having it declared in licenses should be enough tough, DONE
• Developer Information
• Source control system information
  • example: https://central.sonatype.org/publish/requirements/#scm-information