Remove form based editing from Ecore model editor

[ndoschek]

Currently, the "Theia model editor" is implemented as a prototype and offers a tree as well as a form editor.
We might consider to implement the tree editor without the form based editor as we already have the property view capability.
This feature is not yet available in the tree editor itself, but on the roadmap: eclipse-emfcloud/theia-tree-editor#22.

In that case, we may consider to enable to implement a selection forwarder of the tree selection to be able to handle tree node selections in the property-view.

[JonasHelming]

I actually would vote for keeping the form based editor, I believe it adds value

[CamilleLetavernier]

We may keep the editor, but it would also be nice to have the tree-widget as a Theia view. Currently, we have to open the entire Tree + Form editor, which takes a lot of extra space, and is not as flexible as a separate Properties + Tree view that you can place around the diagram editor