Use global SystemParameters.FocusVisualStyleKey insead of per control FocusVisual style

**Is your feature request related to a problem? Please describe.**
A [bunch](https://github.com/MaterialDesignInXAML/MaterialDesignInXamlToolkit/search?q=FocusVisual) of control styles define a `FocusVisual` style that is used as a setter value for `FocusVisualStyle` property and there is no way to customize them other than creating custom styles for each control and setting `FocusVisualStyle` to your own style.

Most of them are identical:
https://github.com/MaterialDesignInXAML/MaterialDesignInXamlToolkit/blob/2a38b2c70a5e851bc5b926e0641c1848e719d86b/MaterialDesignThemes.Wpf/Themes/MaterialDesignTheme.Button.xaml#L17-L30

Only `OptionMarkFocusVisual` seems to have different Margin:
https://github.com/MaterialDesignInXAML/MaterialDesignInXamlToolkit/blob/2a38b2c70a5e851bc5b926e0641c1848e719d86b/MaterialDesignThemes.Wpf/Themes/MaterialDesignTheme.RadioButton.xaml#L13-L25

I think the `FocusVisualStyle` should be globally defined with `SystemParameters.FocusVisualStyleKey` in App.xaml:
```xaml

<Setter Property="Control.Template">
...
</Setter>

```

If Margin is all that needs to be changed for some controls then maybe a `FocusVisualAssist.Margin` could be added to allow those controls to have their own `FocusVisualStyle`:
```xaml

<Setter Property="materialDesign:FocusVisualAssist.Margin" Value="0,0,0,0"/>

```

Start with the FocusVisual definitions in MaterialDesignTheme.Button.xaml and MaterialDesignTheme.RadioButton.xaml, then inspect the proposed App.xaml use of SystemParameters.FocusVisualStyleKey. Compare the shared template and the differing OptionMark margin, and consider the existing FocusVisualAssist path. Done means the duplicated per-control styles are replaced or consolidated without changing focus rendering.

Written by the indexing model from the issue text.