Add support for LoadingSpinner label

[philippjfr]

• Fixes impossible to display a title with Trend widget #5165
• Adds support for giving LoadingSpinner a label

[codecov]

Codecov Report

Merging #5194 (ddf1df4) into main (eaf5853) will increase coverage by 11.17%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##             main    #5194       +/-   ##
===========================================
+ Coverage   72.62%   83.80%   +11.17%     
===========================================
  Files         274      274               
  Lines       39583    39593       +10     
===========================================
+ Hits        28749    33182     +4433     
+ Misses      10834     6411     -4423     

Flag	Coverage Δ
ui-tests	40.36% <23.07%> (?)
unitexamples-tests	73.77% <100.00%> (+1.14%)	⬆️

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Impacted Files	Coverage Δ
panel/tests/widgets/test_base.py	98.36% <ø> (ø)
panel/widgets/indicators.py	73.68% <100.00%> (+0.40%)	⬆️

... and 72 files with indirect coverage changes

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