Use widget_type for Param widget override

[philippjfr]

Fixes #1574

[codecov]

Codecov Report

Merging #1614 into master will decrease coverage by 0.00%.
The diff coverage is 80.00%.

@@            Coverage Diff             @@
##           master    #1614      +/-   ##
==========================================
- Coverage   85.40%   85.39%   -0.01%     
==========================================
  Files         147      147              
  Lines       16578    16580       +2     
==========================================
+ Hits        14158    14159       +1     
- Misses       2420     2421       +1     

Impacted Files	Coverage Δ
panel/param.py	89.80% <80.00%> (-0.17%)	⬇️

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