Allow collapsing overflow area of ToolBar

还没有人认领这个 Issue。

**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.

首先定位 issue 中引用的 ToolBar 模板和 ToolBarOverflowButtonVisibilityConverter，然后检查溢出区域在 XAML 中的显示方式。添加一个可配置的 XAML 行为，用于折叠该区域，同时不改变其他对 Grid 的使用，并验证带有和不带溢出的工具栏是否按预期渲染。

由索引模型根据 Issue 内容生成。