Add support for displaying details on arbitrary element kinds

[pcdavid]

Currently the "Details" view only supports showing properties of semantic elements.
This assumption is hard-coded in the frontend in PropertiesWebSocketContainerMachine which has:

      isSelectionUnsupported: (_, event) => {
        const { selection } = event as SwitchSelectionEvent;
        return !selection || !selection.kind.startsWith('siriusComponents://semantic');
      }

There is no reason for the frontend to care about the kind of element selected and restrict which are supported.
It should be up to the backend to decide if it has details to show for a particular selection (including a multiple selection).
For example it could make sense to display some properties/metadata on representations, or more generally on custom kinds of items specific to a particular application.