Micro optimizations for Viewable

[philippjfr]

Some small optimizations on Viewable that do add up when you create a lot of components:

• Cache Design instances created internally (99% of the time the Design can and should be shared across all Viewable instances)
• Do not run stop_loading if loading=False on instantiation (this is pointless to run on instantiation)
• Small optimization avoiding .param.trigger() on ChatMessage

[codecov]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (cd22c2a) 84.64% compared to head (0a082d9) 40.62%.
Report is 2 commits behind head on main.

❗ Current head 0a082d9 differs from pull request most recent head f12b633. Consider uploading reports for the commit f12b633 to get more accurate results

Files	Patch %	Lines
panel/chat/message.py	0.00%	3 Missing ⚠️
panel/viewable.py	80.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #6074       +/-   ##
===========================================
- Coverage   84.64%   40.62%   -44.03%     
===========================================
  Files         295      295               
  Lines       44003    43994        -9     
===========================================
- Hits        37248    17871    -19377     
- Misses       6755    26123    +19368     

Flag	Coverage Δ
ui-tests	40.62% <61.53%> (+<0.01%)	⬆️
unitexamples-tests	?

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