Introduce reference resolution and completion mechanism for elements
- Ensure we send the global id as part of each identifiable element
- Attach reference data to GLSP elements for property view
- Replace specific 'requestDiagramNodeEntityModel' method
- Replace specific 'findRootReferenceName' method

Create Form for Relationship data
- Unify form for property view and editor (also for Entity)
- Use MUI components consistently and add theming for them
- Remove custom stylesheets
- Remove dependency to 'react-tabs' as it is no longer used

Refactorings:
- Rename 'ExternalId' to 'GlobalId'
- Ensure IDs do not contain any invalid characters
- Fix issue with attribute serialization for relationships

…rs there).

…cked up on mac.

Added example theia settings file to disable autosave in this workspace.

- Remove @mui/lab
- Introduce dirty state into the Header part for properties
- Disable attribute move up/down when appropriate
- Warn user before unsaved changes are lost
- Further unify editor and property widget for save mechanism
- Overwrite Theia property widget to delegate Save command (Ctrl+S)