feat: add avalonia DefaultFocusAdorner style.

Author: NaBian

src/Avalonia/HandyControl_Avalonia/Themes/Styles/AdornerLayer.axaml

@@ -0,0 +1,20 @@
+<ResourceDictionary xmlns="https://github.com/avaloniaui"
+                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+                    x:ClassModifier="internal">
+
+    <ControlTheme x:Key="{x:Type AdornerLayer}"
+                  TargetType="AdornerLayer">
+        <Setter Property="DefaultFocusAdorner">
+            <FocusAdornerTemplate>
+                <Rectangle Margin="0"
+                           Opacity="0.6"
+                           Stroke="{DynamicResource SecondaryBorderBrush}"
+                           RadiusX="4"
+                           RadiusY="4"
+                           StrokeDashArray="1,1"
+                           StrokeThickness="2" />
+            </FocusAdornerTemplate>
+        </Setter>
+    </ControlTheme>
+
+</ResourceDictionary>

src/Avalonia/HandyControl_Avalonia/Themes/Styles/MergedStyles.axaml

@@ -15,6 +15,7 @@
                 <MergeResourceInclude Source="avares://HandyControl/Themes/Basic/Brushes.axaml" />
                 <MergeResourceInclude Source="avares://HandyControl/Themes/Basic/Converters.axaml" />
 
+                <MergeResourceInclude Source="avares://HandyControl/Themes/Styles/AdornerLayer.axaml" />
                 <MergeResourceInclude Source="avares://HandyControl/Themes/Styles/DataValidationErrors.axaml" />
                 <MergeResourceInclude Source="avares://HandyControl/Themes/Styles/MenuFlyoutPresenter.axaml" />
                 <MergeResourceInclude Source="avares://HandyControl/Themes/Styles/PopupRoot.axaml" />