Created Unassigned: Alt key underscores not showing up within AvalonDock control [16479]

Say I have a wpf button such as:

```
<Button Command="{Binding CloseConfirmCommand}" Content="_Close"></Button>
```

When this button is in a control outside of AvalonDock, everything works as expected. But, when the button is in a control inside AvalonDock, pressing down the Alt key does not show the underlined text. The command does work (Alt+c closes the control).

This is an example of how I configured AvalonDock, essentially using document panes with a bunch of custom styling (nothing that reset RecognizedAccessKey):

```
<avalonDock:DockingManager x:Name="tabDesigner" DocumentsSource="{Binding Items}" ScrollViewer.CanContentScroll="True" PreviewMouseRightButtonDown="OnPreviewMouseRightButtonDown" LayoutUpdated="tabDesigner_LayoutUpdated" PreviewMouseDown="tabDesigner_PreviewMouseDown" ActiveContentChanged="tabDesigner_SelectionChanged">
<avalonDock:DockingManager.LayoutItemContainerStyle>
<Style TargetType="{x:Type avalonDockControls:LayoutItem}" BasedOn="{StaticResource DocumentItem}"/>
</avalonDock:DockingManager.LayoutItemContainerStyle>
<avalonDock:DockingManager.DocumentPaneControlStyle>
<Style TargetType="{x:Type avalonDockControls:LayoutDocumentPaneControl}" BasedOn="{StaticResource DocumentPane}"/>
</avalonDock:DockingManager.DocumentPaneControlStyle>
<avalonDockLayout:LayoutRoot>
<avalonDockLayout:LayoutPanel Orientation="Horizontal">
<avalonDockLayout:LayoutDocumentPane/>
</avalonDockLayout:LayoutPanel>
</avalonDockLayout:LayoutRoot>
</avalonDock:DockingManager>
```

I'm currently using 2.0.1746, but quickly tried 2.0.2000 with the same issue.

Has anyone come across a similar issue? Any ideas?