Do not process events related to fake Template root

[philippjfr]

Panel was built around the idea that everything has a single root model which contains all the other models. Templates came along and broke that assumption, because it allows you to insert multiple roots at different points of an Jinja template.

To deal with that we create a fake root model, which makes things like cross-root linking of axes and JS links across roots work. However the rest of the code was not aware this thing was a fake, so it was creating duplicate models for it and keeping them all updated. So bad news is, that’s a horrific bug, good news is everything that was using templates is about to get 2x faster.

[codecov]

Codecov Report

Merging #1216 into master will decrease coverage by 0.02%.
The diff coverage is 72.72%.

@@            Coverage Diff             @@
##           master    #1216      +/-   ##
==========================================
- Coverage   87.44%   87.41%   -0.03%     
==========================================
  Files         106      106              
  Lines       12607    12619      +12     
==========================================
+ Hits        11024    11031       +7     
- Misses       1583     1588       +5     

Impacted Files	Coverage Δ
panel/widgets/base.py	80.95% <0.00%> (-1.98%)	⬇️
panel/pane/base.py	89.00% <50.00%> (-0.40%)	⬇️
panel/layout.py	84.61% <75.00%> (-0.16%)	⬇️
panel/viewable.py	77.38% <87.50%> (ø)
panel/io/state.py	90.19% <100.00%> (+0.19%)	⬆️
panel/template.py	66.93% <100.00%> (+0.26%)	⬆️

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