BarPlugin YAML config

[dalyIsaac]

• Added a declarative config for the BarPlugin.
• To improve testability, BarComponent is now an abstract record. This doesn't change the behavior of the bar.
• Added missing docs for the window backdrop

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 84.35374% with 23 lines in your changes missing coverage. Please review.

Project coverage is 80.94%. Comparing base (aa10103) to head (de687f4). Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/Whim.Yaml/YamlBarPluginLoader.cs	90.90%	8 Missing and 2 partials :warning:
src/Whim.Bar/BarWindow.xaml.cs	0.00%	5 Missing :warning:
src/Whim.Bar/BarPlugin.cs	60.00%	2 Missing :warning:
...c/Whim.Bar/ActiveLayout/ActiveLayoutWidget.xaml.cs	50.00%	1 Missing :warning:
src/Whim.Bar/Battery/BatteryWidget.xaml.cs	50.00%	1 Missing :warning:
src/Whim.Bar/DateTime/DateTimeWidget.xaml.cs	66.66%	1 Missing :warning:
...Whim.Bar/FocusedWindow/FocusedWindowWidget.xaml.cs	66.66%	1 Missing :warning:
src/Whim.Bar/Workspace/WorkspaceWidget.xaml.cs	50.00%	1 Missing :warning:
src/Whim.TreeLayout.Bar/TreeLayoutBarPlugin.cs	66.66%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1050 +/- ## ========================================== + Coverage 80.76% 80.94% +0.18% ========================================== Files 281 282 +1 Lines 12158 12273 +115 Branches 1400 1413 +13 ========================================== + Hits 9819 9934 +115 + Misses 2163 2161 -2 - Partials 176 178 +2 ```

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