UiSchema can't hold advanced titleMap for more advanded datalist widgets

The datalist widget on the frontend side requires a titleMap to be filled in in its UI Schema. The default behaviour of the datalist is to support a collection of tuple (named NameValue in component-runtime) made of a technical ID and a display name.

It exists a more advanced usage of the datalist widget, which allows to have several list of elements in the drop down list, separated with a separator and a title. In order to support this, the component-runtime must evolve in order to support something else than a NameValue in the titleMap.

Attached an example of what is supported in the UI with this advanced drop down list, and the related UI Schema fragment in JSON.