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.

Start by locating the ToolBar template and ToolBarOverflowButtonVisibilityConverter referenced in the issue, then inspect how the overflow area is shown in XAML. Add a configurable XAML behavior for collapsing that area without changing other Grid uses, and verify that toolbars with and without overflow render as expected.

Written by the indexing model from the issue text.