Add generic builder API for Graph model elements

[tortmayr]

• Add abstract extensible builder classes for all base graph model elements
• Add implementations for the default model elements
• Migrate existing builder for workflow example to new API

[planger]

@tortmayr this time there seems to be an actual build error: https://travis-ci.org/eclipsesource/graphical-lsp/jobs/577251503 (compilation error)

[tortmayr]

@planger Fixed. I added an additional minor change which improves the ServerStatus API