Upgrade to Eclipse 2020-03

[lwrage]

• Update setup to use Eclipse 2020-03 repository
• Switch to basic Eclipse product
• Add required modeling features to setup
• Update versions of used features where necessary (Sirius)
• Get Xtext from separate repository
• Update target and product
• Update site URLs in repository pom
• Update p2mirror script
• Update mirrored repositories on build server
• Update developer setup instructions
• Post instructions on google group

[lwrage]

Needed to update EASE to 0.7.0