Add Streamz pane with ReplacementPane base class

[philippjfr]

Factors out components from ParamMethod class into a ReplacementPane abstract baseclass which is then used as the basis for a streamz.Stream class which allows streaming all kinds of objects in addition to the objects DataFrame like streamz objects supported by the DataFrame pane.

[codecov-io]

Codecov Report

Merging #767 into master will decrease coverage by 0.14%.
The diff coverage is 61.01%.

@@            Coverage Diff             @@
##           master     #767      +/-   ##
==========================================
- Coverage   69.58%   69.44%   -0.15%     
==========================================
  Files         102      103       +1     
  Lines       10211    10259      +48     
==========================================
+ Hits         7105     7124      +19     
- Misses       3106     3135      +29

Impacted Files	Coverage Δ
panel/pane/__init__.py	34.78% <0%> (-1.59%)	⬇️
panel/tests/test_io.py	100% <100%> (ø)	⬆️
panel/links.py	65.57% <100%> (+0.69%)	⬆️
panel/tests/test_viewable.py	100% <100%> (ø)	⬆️
panel/tests/test_param.py	99.84% <100%> (-0.01%)	⬇️
panel/pane/streamz.py	32.35% <32.35%> (ø)
panel/param.py	76.61% <58.33%> (+0.75%)	⬆️
panel/tests/util.py	83.6% <60%> (-2.11%)	⬇️
panel/pane/base.py	60% <64.28%> (+1.66%)	⬆️

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