SceneKey class sealed

What a key names.

A key is what a SceneNode.bindings entry is filed under, what the read plane gets and sets by, and what a motion track is named. There are two things it can be: a property of the node, or one PART of a property whose value has named parts — a side of a padding, an argument of the child a node stands for.

It used to be four, because a property that the file spells its own way could not be a table row and got an ad-hoc key instead — style under a reserved name, args. under a prefix — and a side, which was nobody's, got no answer at all and was silently dropped. SceneProp.byHand is what lets those be ordinary rows, and the four collapsed into these two.