Make PageText.sortPosition() sort order deterministic.

[peterwilliams97]

This is a fix for #152

The sort order is now deterministic and the ordering is explained in the source code.

A test case shows how the sorting works.

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This change is 

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Codecov Report

❗ No coverage uploaded for pull request base (development@2d20058). Click here to learn what that means.
The diff coverage is 89.47%.

@@              Coverage Diff               @@
##             development     #153   +/-   ##
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  Coverage               ?   57.14%           
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  Files                  ?      187           
  Lines                  ?    33719           
  Branches               ?        0           
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  Hits                   ?    19270           
  Misses                 ?    11954           
  Partials               ?     2495

Impacted Files	Coverage Δ
extractor/text.go	65.48% <89.47%> (ø)

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[gunnsth]

Seems to me like the previous algorithm should work with small change. That one is preferable due to its simplicity. If that is not the case, the rationale for the added complexity should be clarified.

[CLAassistant]

All committers have signed the CLA.

[gunnsth]

LGTM