Add LinearGauge indicator

[philippjfr]

A LinearGauge provides a much more compact value indicator than Dial or Gauge.

• Add tests
• Make colors handling consistent with Gauge and Dial
• Improve horizontal support

[jbednar]

That's useful, but I'd think that in a bank of them the current value of it won't stand out much, while I'd want higher values to be very visible in many cases. The usual way that's done is to dim the alpha of the background for the areas above the current value, potentially to zero.

In general since there is really only one value being displayed, one will often want this to be very small, so it would be good if the defaults weren't assuming this needs font and tick sizes like a curve or scatterplot...

Also, the colors here seem backward if that's a failure rate. :-)

[philippjfr]

Agree with everything you said. Total work in progress for now.

[codecov]

Codecov Report

Merging #3222 (c9feea1) into master (2fbd79b) will decrease coverage by 0.38%.
The diff coverage is 22.22%.

@@            Coverage Diff             @@
##           master    #3222      +/-   ##
==========================================
- Coverage   83.34%   82.96%   -0.39%     
==========================================
  Files         198      198              
  Lines       26791    26970     +179     
==========================================
+ Hits        22329    22375      +46     
- Misses       4462     4595     +133     

Impacted Files	Coverage Δ
panel/tests/widgets/test_base.py	100.00% <ø> (ø)
panel/widgets/__init__.py	100.00% <ø> (ø)
panel/widgets/indicators.py	72.74% <22.22%> (-19.75%)	⬇️
panel/layout/accordion.py	89.79% <0.00%> (-0.74%)	⬇️
panel/pane/idom.py	27.20% <0.00%> (ø)
panel/pane/plot.py	58.37% <0.00%> (ø)
panel/pane/vega.py	83.04% <0.00%> (ø)
panel/pane/alert.py	100.00% <0.00%> (ø)
panel/pane/image.py	80.09% <0.00%> (ø)
panel/pane/media.py	97.72% <0.00%> (ø)
... and 12 more

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