Refactor Page and LayerPage

[dreua]

Further improvement ideas:

• The whole thing would ideally become a @dataclass which would generate the __init__ and __repr__ methods automatically, lowering manual maintenance, repetition and risk of error. The size calculation could happen in __post_init__. TBD: Would it be reasonable to require Python >= 3.7? I'd hope so since it just reached its end of life.
• Why are there two nearly identical Page classes? Pretty sure we should derive one from the other or from one common ancestor.

Originally posted by @dreua in #914 (review)