Display the contents of a Portal in the Representations view

[pcdavid]

When a Portal is selected, the Representations view should display a tree of all the representations embedded in the portal.
It should be a tree to also show the internals of any sub-portals.
Selecting a node in this tree should open the corresponding representation in "full mode".

Note that the Representations view on a semantic element shows a list of the representations attached to this element, where each item (representation) can be selected but also deleted. This is not possible when using a tree widget, where tree nodes do not support a delete action (or any other).