Added editing features to Langium-based example

[spoenemann]

These operations are now possible with a side-by-side view of the Langium-based States editor and a Sprotty view.

• Select a state or transition and press delete 👉 the respective text range is deleted
• Double-click a state name 👉 a dialog appears where you can enter a new name
• Double-click a transition event 👉 a dialog appears where you can choose another event
• Hover on the diagram background 👉 a pop-up appears with buttons to create a new state or event

Does not work, to be fixed later:

• Drag the little triangle on the right side of a node to create a new transition
• Select a transition, then drag the source or target handle to change the source or target state

[dhuebner]

Works really good!
Small things I noticed:

• In the Xtext example the hover menu is not rendered properly (is empty)
• Also in the Xtext example LS throws java.lang.IllegalArgumentException: Unknown action kind: languageLabelEdit. Editing works as expected.
• If possible do not render hover menu in the View example as it doesn't work

[spoenemann]

I'd remove the Xtext example as we can't maintain it anymore.