Display access key in Buttons.

Set `RecognizesAccessKey` on button content presenter.

samples/ControlCatalog/Pages/ButtonPage.xaml

@@ -10,7 +10,7 @@
                 HorizontalAlignment="Center"
                 Spacing="16">
       <StackPanel Orientation="Vertical" Spacing="8" Width="200">
-        <Button>Standard XAML Button</Button>
+        <Button>Standard _XAML Button</Button>
         <Button Foreground="White">Foreground</Button>
         <Button Background="{DynamicResource SystemAccentColor}">Background</Button>
         <Button IsEnabled="False">Disabled</Button>

src/Avalonia.Themes.Default/Button.xaml

@@ -17,6 +17,7 @@
                           ContentTemplate="{TemplateBinding ContentTemplate}"
                           Content="{TemplateBinding Content}"
                           Padding="{TemplateBinding Padding}"
+                          RecognizesAccessKey="True"
                           TextBlock.Foreground="{TemplateBinding Foreground}"
                           HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}"
                           VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}"/>

src/Avalonia.Themes.Fluent/Controls/Button.xaml

@@ -34,6 +34,7 @@
                           Content="{TemplateBinding Content}"
                           ContentTemplate="{TemplateBinding ContentTemplate}"
                           Padding="{TemplateBinding Padding}"
+                          RecognizesAccessKey="True"
                           HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}"
                           VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}" />
       </ControlTemplate>