Add App.update method.

[BrianPugh]

Public method to allow the combining of apps.

Helps address some issues raised in #276

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.84%. Comparing base (34717e3) to head (e24be40).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #281   +/-   ##
=======================================
  Coverage   93.83%   93.84%           
=======================================
  Files          26       26           
  Lines        3003     3005    +2     
  Branches      633      633           
=======================================
+ Hits         2818     2820    +2     
  Misses         90       90           
  Partials       95       95           

Flag	Coverage Δ
unittests	93.84% <100.00%> (+<0.01%)	⬆️

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