Use global SystemParameters.FocusVisualStyleKey insead of per control FocusVisual style

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**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"/>

```

Inizia con le definizioni di FocusVisual in MaterialDesignTheme.Button.xaml e MaterialDesignTheme.RadioButton.xaml, quindi esamina l’uso proposto di SystemParameters.FocusVisualStyleKey in App.xaml. Confronta il template condiviso e il margine diverso di OptionMark, e considera il percorso FocusVisualAssist esistente. Il lavoro è completo quando gli stili duplicati per controllo vengono sostituiti o consolidati senza modificare la resa del focus.

Scritto dal modello di indicizzazione a partire dal testo della issue.