Add memoization for invoking callbacks

[b-yogesh]

Closes #112

This PR also closes xcube-dev/xcube-viewer#490

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.53%. Comparing base (e18dc5c) to head (7a49490).
Report is 17 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #110      +/-   ##
==========================================
+ Coverage   89.39%   92.53%   +3.13%     
==========================================
  Files          84       85       +1     
  Lines        1443     1460      +17     
  Branches      217      220       +3     
==========================================
+ Hits         1290     1351      +61     
+ Misses        124       91      -33     
+ Partials       29       18      -11     

Flag	Coverage Δ
frontend	84.13% <100.00%> (+7.07%)	⬆️

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[forman]

• Why does this work? Please comment.
• Global variable used instead of local memoization
• Why time-consuming JSON conversion instead of suggested shallow comparison
• Missing unit test

This seems to be an insufficient solution as the key used into the map of memoized values is the value serialized itself. But the key should be an ID created from the propertyRef and the value should be the array of input values.