Batch ParamFunction events

[philippjfr]

ParamFunction was needlessly creating multiple watch callbacks.

[codecov]

Codecov Report

Merging #1284 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1284   +/-   ##
=======================================
  Coverage   87.77%   87.77%           
=======================================
  Files         124      124           
  Lines       13477    13480    +3     
=======================================
+ Hits        11829    11832    +3     
  Misses       1648     1648           

Impacted Files	Coverage Δ
panel/param.py	90.63% <100.00%> (+0.06%)	⬆️

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