feat: add get_app class method to Application

[tlambert03]

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[codecov]

Codecov Report

Merging #48 (c747d45) into main (6f34971) will decrease coverage by 0.05%.
The diff coverage is 75.00%.

❗ Current head c747d45 differs from pull request most recent head 03d8f73. Consider uploading reports for the commit 03d8f73 to get more accurate results

@@            Coverage Diff             @@
##             main      #48      +/-   ##
==========================================
- Coverage   99.69%   99.63%   -0.06%     
==========================================
  Files          31       31              
  Lines        1656     1659       +3     
==========================================
+ Hits         1651     1653       +2     
- Misses          5        6       +1     

Impacted Files	Coverage Δ
src/app_model/_app.py	98.52% <75.00%> (-1.48%)	⬇️

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