Define the visible-page sort function, used in getVisibleElements, once

[Snuffleupagus]

When scrolling through long documents, especially when using spread modes and/or wrapped scrolling, the visible-page sorting can be invoked a lot.
Hence it seems like a good idea to define the sort function just once, rather than re-creating it for every invocation.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.16%. Comparing base (6694c0e) to head (d8e5a04).

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #21787      +/-   ##
==========================================
- Coverage   90.16%   90.16%   -0.01%     
==========================================
  Files         264      264              
  Lines       67268    67266       -2     
==========================================
- Hits        60652    60649       -3     
- Misses       6616     6617       +1     

Flag	Coverage Δ
browsertest	66.29% <0.00%> (-0.03%)	⬇️
integrationtest	69.37% <100.00%> (+<0.01%)	⬆️
unittest	58.28% <100.00%> (+0.01%)	⬆️
unittestcli	56.81% <100.00%> (+0.01%)	⬆️

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[timvandermeij]

Good idea; thanks!