Provide alternative graph-based view for lazy models

[agarciadom]

Our specialization of Exeed only has a tree-based notation, which is not helpful for flat metamodels (i.e. without containment) like the IFC ones. A graph-based visualization would be more helpful.

[agarciadom]

The visualization could be based on Zest, and should be implemented as an extra tab on the Exeed editor that is initialized lazily (to avoid paying the cost if we don't use it). The visualization should hook up to the Exeed resource-specific customizations: on the first load it'll need to access all the current contents of the resource, without fetching anything extra. The HawkResource customization will have to ask the LazyResolver if a particular reference is pending or not, and skip the pending ones.