Initial support for Spaces

[KitsuneRal]

At least:

• [ ] Distinguish space-rooms (via m.space room "type") - maybe even devise class Space : public Room for that?
• [ ] Add a class for m.space.child state event and respective room list accessors in Room (Space?)
• [ ] Add a class for m.space.parent state event and respective accessors in Room (again, Space* might be a good data type for accessors)