Fix page chooser widget extraction in orderables

[zerolab]

Fixes #573

Prior to v1.1 we explicitly checked that the given edit_handler was a PageChooserPanel. with 3.0+ edit handlers are now panels and the mechanism for extraction is slightly different

[codecov-commenter]

Codecov Report

Merging #584 (9556d7c) into main (e359830) will increase coverage by 0.04%.
The diff coverage is 81.25%.

@@            Coverage Diff             @@
##             main     #584      +/-   ##
==========================================
+ Coverage   91.55%   91.60%   +0.04%     
==========================================
  Files          47       47              
  Lines        3898     3906       +8     
  Branches      592      593       +1     
==========================================
+ Hits         3569     3578       +9     
- Misses        185      186       +1     
+ Partials      144      142       -2     

Impacted Files	Coverage Δ
wagtail_localize/views/edit_translation.py	85.85% <62.50%> (+0.41%)	⬆️
wagtail_localize/test/models.py	97.78% <100.00%> (+0.07%)	⬆️
wagtail_localize/models.py	95.42% <0.00%> (-0.15%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update e359830...9556d7c. Read the comment docs.