Implemented __signature__ for Viewable objects

[philippjfr]

Replaces the param docstring signature with a __signature__ attribute, which should allow for tab-completion in newer versions of IPython. The new signature is applied on extension load as long as pn.config.apply_signature is set.

Additionally this enforces strict rules on what Pane, Widget, and Panel component signatures must look like during testing. This ensures no new components are added without considering how the signature code affects them.

[codecov]

Codecov Report

Merging #1029 into master will decrease coverage by 0.29%.
The diff coverage is 61.72%.

@@            Coverage Diff            @@
##           master    #1029     +/-   ##
=========================================
- Coverage   85.95%   85.65%   -0.3%     
=========================================
  Files         105      105             
  Lines       11924    12062    +138     
=========================================
+ Hits        10249    10332     +83     
- Misses       1675     1730     +55

Impacted Files	Coverage Δ
panel/__init__.py	100% <100%> (ø)	⬆️
panel/tests/pane/test_base.py	87.5% <100%> (+3.5%)	⬆️
panel/tests/util.py	84.12% <100%> (+0.52%)	⬆️
panel/tests/test_layout.py	100% <100%> (ø)	⬆️
panel/tests/widgets/test_base.py	100% <100%> (ø)	⬆️
panel/tests/test_viewable.py	100% <100%> (ø)	⬆️
panel/widgets/select.py	93.95% <100%> (ø)	⬆️
panel/pane/vtk/vtk.py	53.92% <100%> (ø)	⬆️
panel/config.py	46.66% <8.82%> (-6.03%)	⬇️
panel/io/notebook.py	59.17% <0%> (-2.96%)	⬇️
... and 8 more

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