Properties view for diagram editor

[tortmayr]

We should make use of the SelectionService hook introduced in 1674493 which enables listening to model changes.

For now it's probably best to create a simple json-forms based property view with edit capabilities

see also eclipsesource/graphical-lsp#26

[planger]

I think we can close this issue. Generic property view has been proposed to eclipse-theia/theia#8655 and JSONForms- and model-server-based will live in EMF.Cloud. So there is no need for this ticket here in GLSP anymore.