Ensure Design is applied globally when using global templates

[philippjfr]

Includes a number of optimizations and fixes when applying a Design, specifically:

• Always cache the creation of ImportedStyleSheet
• If an app uses pn.state.template the template's design is applied globally (for the session)
• Design modifiers are only applied once per model (e.g. ensuring that wrapper components have final say on the themes applied to the model)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.72%. Comparing base (62be0e5) to head (70efef8).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6992      +/-   ##
==========================================
- Coverage   81.73%   81.72%   -0.01%     
==========================================
  Files         326      326              
  Lines       48082    48095      +13     
==========================================
+ Hits        39299    39305       +6     
- Misses       8783     8790       +7     

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