Let ILayoutEngines perform custom actions

[dalyIsaac]

This PR adds the ability for layout engines to perform non-standard actions, and still have their resulting layouts be saved to Workspace instances. This was needed for #634, where the SliceLayoutPlugin tries to perform operations on the SliceLayoutEngine, but was unable to save the resulting ILayoutEngine instance to the Workspace.

The PerformCustomAction method can be used by ILayoutEngine implementations to perform non-standard actions.

[codecov]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (ca758ae) 79.73% compared to head (5bdced4) 79.83%.

Files	Patch %	Lines
src/Whim/Layout/LayoutEngineCustomAction.cs	60.00%	2 Missing ⚠️
src/Whim.FloatingLayout/FloatingLayoutEngine.cs	87.50%	0 Missing and 1 partial ⚠️
src/Whim.TestUtils/TestLayoutEngine.cs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #736      +/-   ##
==========================================
+ Coverage   79.73%   79.83%   +0.09%     
==========================================
  Files         179      180       +1     
  Lines        8814     8877      +63     
  Branches      990      996       +6     
==========================================
+ Hits         7028     7087      +59     
- Misses       1672     1675       +3     
- Partials      114      115       +1     

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