Allow collapsing overflow area of ToolBar

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**Is your feature request related to a problem? Please describe.**
A ToolBar always contains the area for the overflow, even there is no overflow at all. I am not sure if this is intended behavior or no. For small toolbars I find this rather annoying.

Here a small example, the overflow part is the lighter area to the right of the ‘B’ button:

In the package code itself I can get achieve my expected result by changing `ToolBarOverflowButtonVisibilityConverter` to return `Visibility.Collapsed` instead of `Visibility.Hidden`. But I guess that could cause issues in other use cases.

**Describe the solution you'd like**
The behavior of the overflow area is configurable in XAML.

**Describe alternatives you've considered**
My current workaround is to collapse the overflow grid using an implicit style:
```XAML

<Setter Property="Visibility" Value="Collapsed" />

```
This works only as long as there are no other uses of Grid.

**Additional context**
This is based on to 4.5.0.

Inizia individuando il modello ToolBar e ToolBarOverflowButtonVisibilityConverter a cui fa riferimento l’issue, quindi esamina come viene visualizzata l’area di overflow in XAML. Aggiungi un comportamento XAML configurabile per comprimere quell’area senza modificare altri utilizzi di Grid e verifica che le toolbar con e senza overflow vengano renderizzate come previsto.

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