Calculate notebook extents lazily

[bollwyvl]

Elevator Pitch

It should be possible to only update extents for a given notebook when the cell count or metadata changes.

Motivation

Currently, the two example notebooks result in _getExtents being called 450 times, with the inner loop executing 17341 times. It's like for a read-only notebook that the outer function could be called twice.

Design Ideas

• calculate when asked and cache
• listen for more signals that invalidate the cache