Implementing "Pages" for versioned groups in Workcraft

1) Collapsible groups (sections)
Add the ability to hide/collapse groups by only showing them as an octagon shape. All connections crossing group borders now become the connections to and from this new shape. Adding new connections to the group is not allowed (at least for now).

2) Sub-page view
Special view from "inside" the group. This doesn't mean a separate model, only the group view with visible nodes inside the group, and the nodes directly connected to the group from outside.
The group itself might store the additional layout information for the nodes inside it.

3) Implementing multi-groups
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