Keyboard navigation for schema tree

[ddeuber]

Navigating through the schema tree is currently only possible by mouse. We should also implement keyboard navigation. For example, the arrow keys could be used to move from the selected node to the next sibling, the parent or to child nodes. What is not clear is how collapsing expanded nodes should be triggered by keyboard.

Code hint: the logic for focusing on the selected node is already implemented in D3SchemaTree.focusOnNode and is triggered when the selectedNode is changed. Therefore, it is enough to call setSelectedNode on the SchemaViewer component to move to another node.