Fix Progress component

[clarasb]

Description

This PR fixes the behaviour of progress components by adding visible support for the components and connecting it to callback execution state.

Progress components can now be hidden with visible=False and shown via callback outputs such as:

Output("progress", "visible")

When a server-side callback declares a progress component’s visible property as an output, chartlets now shows that progress indicator immediately while the callback request is pending. Once the callback completes, the normal callback output is applied, typically hiding the progress component again.

Changes

• Added visible to the shared component state model.
• Made CircularProgress and LinearProgress respect visible={false}.
• Added pending-callback progress handling in invokeCallbacks().
• Added protection for overlapping pending callbacks so a progress component is not hidden too early.
• Added/updated tests for progress visibility and Python serialization.
• Added a demo panel.
• Other changes:
  • Fixed vega_test.py.

Demo Panel

xcube-viewer panels demo

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.75%. Comparing base (f8cf975) to head (0bee733).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #145      +/-   ##
==========================================
+ Coverage   92.53%   93.75%   +1.21%     
==========================================
  Files          90       91       +1     
  Lines        1594     1665      +71     
  Branches      243      262      +19     
==========================================
+ Hits         1475     1561      +86     
+ Misses         99       88      -11     
+ Partials       20       16       -4     

Flag	Coverage Δ
backend	100.00% <100.00%> (ø)
frontend	88.04% <100.00%> (+2.90%)	⬆️

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