Global configuration to always throttle sliders

[hoxbro]

closes #2295

Todo:

• If the implementation is correct or another implementation is needed.
• If config.throttled is the correct/best name for the configuration.
• Add unit test
• Update documentation
• Other?

[codecov]

Codecov Report

Merging #2306 (e2a7855) into master (078c8b2) will increase coverage by 0.42%.
The diff coverage is 96.77%.

@@            Coverage Diff             @@
##           master    #2306      +/-   ##
==========================================
+ Coverage   83.68%   84.10%   +0.42%     
==========================================
  Files         183      181       -2     
  Lines       22018    22064      +46     
==========================================
+ Hits        18425    18557     +132     
+ Misses       3593     3507      -86     

Impacted Files	Coverage Δ
panel/widgets/slider.py	75.18% <71.42%> (-0.32%)	⬇️
panel/config.py	51.92% <100.00%> (+0.14%)	⬆️
panel/tests/widgets/test_input.py	100.00% <100.00%> (ø)
panel/tests/widgets/test_slider.py	100.00% <100.00%> (ø)
panel/widgets/input.py	96.74% <100.00%> (+0.32%)	⬆️
...nel/tests/template/fast/test_fast_list_template.py	87.50% <0.00%> (-6.25%)	⬇️
panel/tests/template/test_vanilla_manual.py	86.95% <0.00%> (-2.41%)	⬇️
...nel/tests/template/fast/test_fast_grid_template.py	95.83% <0.00%> (-2.09%)	⬇️
panel/tests/template/test_manual.py	61.61% <0.00%> (-1.39%)	⬇️
panel/reactive.py	76.34% <0.00%> (ø)
... and 10 more

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[philippjfr]

Thanks, I'm happy with this.