feat: combine app model

[tlambert03]

top level centralized Application that contains all the registries

[codecov]

Codecov Report

Merging #9 (9634881) into main (c8e67fe) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main       #9      +/-   ##
==========================================
+ Coverage   99.39%   99.42%   +0.02%     
==========================================
  Files          17       19       +2     
  Lines         658      691      +33     
==========================================
+ Hits          654      687      +33     
  Misses          4        4              

Impacted Files	Coverage Δ
src/app_model/__init__.py	100.00% <100.00%> (ø)
src/app_model/_app.py	100.00% <100.00%> (ø)
src/app_model/registries/_commands.py	92.45% <100.00%> (-0.77%)	⬇️
src/app_model/registries/_keybindings.py	100.00% <100.00%> (ø)
src/app_model/registries/_menus.py	100.00% <100.00%> (ø)
src/app_model/registries/_register.py	100.00% <100.00%> (ø)
src/app_model/types/__init__.py	100.00% <100.00%> (ø)
src/app_model/types/_icon.py	100.00% <100.00%> (ø)
src/app_model/types/_keybinding.py	100.00% <100.00%> (ø)
src/app_model/types/_menu.py	100.00% <100.00%> (ø)
... and 1 more

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