Notes on spatial transforms & spatial topology (#4908)

### What

* Fixes #4655

As discussed on Slack, noted down what we decided on spatial transforms.
Extended it a bit from there

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design/spatial_transforms.md

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+# Spatial Transforms
+
+Spatial transforms are transforms that apply the spatial 2D & 3D space views.
+This includes affine 2D/3D transforms as well as camera projections.
+
+Any transform component that is logged a path `parent/entity` it describes the
+transform between `parent` to `parent/entity`.
+
+
+## Topology
+We infer a `SpatialTopology` from these transforms.
+As we get more information about a scene the topology changes, but all changes are irreversible.
+In practical terms this means that once a pinhole is logged we'll always assume that everything under
+the entity path of that camera is in 2D space.
+
+The spatial topology is used to determine which (spatial) visualizers can be used in which contents.
+A 2D visualizer can only ever be applied to an entity when there is a valid transformation
+along the path of the entity to the space view's origin.
+
+Examples for invalid transformation paths are:
+* mismatched start space
+  * 2D content can not be added to a 3D space and vice versa
+* several projections (a pinhole can only be applied once)
+* explicit space disconnect 
+
+## `DisconnectedTransform` (former `DisconnectedSpace`)
+Disconnected transform is a special transform that forbids any transformation path
+from an entity to its parent and vice versa.
+As such it creates an explicit break in the topology.
+
+Like any other topological break, it is permanent. Once logged, the resulting subspaces can no longer be fused.
+
+## Null transforms
+Null transforms are handled like identity transforms, the same as not logging a transform at all.