Add busy/loading indicators and enable on Template

[philippjfr]

Adds two boolean indicators which can be used independently or in one of the default templates.

BooleanStatus

LoadingSpinner

Template

Here we can see the indicator on the right of the template header.

[MarcSkovMadsen]

Awesome.

Will it be possible to easily set a custom color?

In general it would be nice to set a custom color for the different button_type, booleanstatus_type, loadingspinner_type etc.. In an organisation there is normally some kind of branding guide you need to follow.

It would be nice to set that style in one place instead of many places. Setting it in CSS if it is easy would also be a good solution. In some of the material frameworks you can set :root settings like --primary-color in CSS and then everything just changes. That is nice. With Panel right now it's not easy.

[philippjfr]

Will it be possible to easily set a custom color?

No, this is currently not supported and can't be easily supported until we improve the way CSS is handled and built. I have plans to switch to something like Sass and compile the CSS properly but for now defining preset colors is the best we can do.

[philippjfr]

In some of the material frameworks you can set :root settings like --primary-color in CSS and then everything just changes.

That is definitely going to be part of that CSS refactoring work.

[codecov]

Codecov Report

Merging #1493 into master will increase coverage by 0.22%.
The diff coverage is 88.79%.

@@            Coverage Diff             @@
##           master    #1493      +/-   ##
==========================================
+ Coverage   86.09%   86.31%   +0.22%     
==========================================
  Files         141      142       +1     
  Lines       15656    15766     +110     
==========================================
+ Hits        13479    13609     +130     
+ Misses       2177     2157      -20     

Impacted Files	Coverage Δ
panel/template/base.py	69.39% <68.29%> (+11.60%)	⬆️
panel/io/state.py	59.63% <100.00%> (+2.75%)	⬆️
panel/tests/test_template.py	97.40% <100.00%> (+1.65%)	⬆️
panel/viewable.py	70.30% <100.00%> (+0.52%)	⬆️
panel/widgets/indicators.py	100.00% <100.00%> (ø)

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