Additional fixes to param API usage

[jlstevens]

Further updates to param usage in panel. Please don't merge yet as there may be more that I find as I fix the warnings found by the holoviews tests!

[codecov]

Codecov Report

Merging #998 into master will increase coverage by 0.22%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #998      +/-   ##
==========================================
+ Coverage   85.85%   86.08%   +0.22%     
==========================================
  Files          99       99              
  Lines       11301    11426     +125     
==========================================
+ Hits         9703     9836     +133     
+ Misses       1598     1590       -8

Impacted Files	Coverage Δ
panel/pane/holoviews.py	84.55% <100%> (ø)	⬆️
panel/config.py	53.08% <100%> (ø)	⬆️
panel/widgets/base.py	83.11% <100%> (ø)	⬆️
panel/links.py	84.14% <100%> (ø)	⬆️
panel/widgets/select.py	94.52% <0%> (+0.35%)	⬆️
panel/param.py	94.02% <0%> (+3.49%)	⬆️

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