Frames produced by block nodes are now always treated as exactly one per
given region and a frame must not be larger than its respective region.
Any overflow must be handled internally. This means that stack and grid
don't need to search for fitting regions anymore, since the child has
already does that for them. This commit further moves stack spacing into
a new `SpacingNode`.
The current inline alignment is very broken and leads to lots of subtle weirdness. Getting rid of it simplifies the stack's interface a lot. At a later point either:
- inline alignment will be added back in a better way, or
- all nodes will be able to expand or align themselves, meaning that the stack's children take care of their alignment
