Add a number of ValueIndicators

[philippjfr]

Adds a number of ValueIndicator which indicate some numeric value, useful for BI dashboard:

• Number
• Dial
• Gauge

[codecov]

Codecov Report

Merging #1528 into master will increase coverage by 0.06%.
The diff coverage is 93.30%.

@@            Coverage Diff             @@
##           master    #1528      +/-   ##
==========================================
+ Coverage   85.72%   85.79%   +0.06%     
==========================================
  Files         145      147       +2     
  Lines       15996    16997    +1001     
==========================================
+ Hits        13713    14582     +869     
- Misses       2283     2415     +132     

Impacted Files	Coverage Δ
panel/tests/widgets/test_base.py	100.00% <ø> (ø)
panel/widgets/indicators.py	93.24% <91.95%> (-6.76%)	⬇️
panel/tests/test_docs.py	100.00% <100.00%> (ø)
panel/tests/widgets/test_indicators.py	100.00% <100.00%> (ø)
panel/widgets/__init__.py	100.00% <100.00%> (ø)
panel/widgets/misc.py	92.66% <100.00%> (-0.42%)	⬇️
panel/depends.py	50.00% <0.00%> (-13.64%)	⬇️
panel/widgets/button.py	82.79% <0.00%> (-2.92%)	⬇️
panel/compiler.py	3.77% <0.00%> (-0.88%)	⬇️
setup.py	0.00% <0.00%> (ø)
... and 16 more

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