• I have checked that this feature has not yet been suggested by someone else.

Expected behavior

The diagram panel has a new icon that shows a tree representing the content of the diagram when clicked.
Elements in the tree can be checked, and a split button is available to apply some tools on the selected elements (e.g. hide, show, collapse).

A checkbox is available to (un) select all the elements in the tree at once.

The interface should look like

The tree and the diagram should be synchronized: an update in the diagram should update the content of the tree (in particular the icons next to each tree node item), and executing a tool from the form view should udpate the diagram accordingly.

This feature is a particular implementation of #3428 with a FormDescription initialized from the content of a diagram representation.

Cuttings backs

• The split button should only support actions that have an associated event handler. In particular, accessing DiagramServices from the form is out of the scope of this issue.