feat: add avalonia Separator style.

Author: NaBian

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

@@ -21,6 +21,7 @@
                 <MergeResourceInclude Source="avares://HandyControl/Themes/Styles/TransitioningContentControl.axaml" />
                 <MergeResourceInclude Source="avares://HandyControl/Themes/Styles/PopupRoot.axaml" />
                 <MergeResourceInclude Source="avares://HandyControl/Themes/Styles/PathIcon.axaml" />
+                <MergeResourceInclude Source="avares://HandyControl/Themes/Styles/Separator.axaml" />
                 <MergeResourceInclude Source="avares://HandyControl/Themes/Styles/ToolTip.axaml" />
                 <MergeResourceInclude Source="avares://HandyControl/Themes/Styles/TextBlock.axaml" />
                 <MergeResourceInclude Source="avares://HandyControl/Themes/Styles/ContentControl.axaml" />

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

@@ -0,0 +1,25 @@
+<ResourceDictionary xmlns="https://github.com/avaloniaui"
+                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+                    x:ClassModifier="internal">
+
+    <ControlTheme x:Key="{x:Type Separator}"
+                  TargetType="Separator">
+        <Setter Property="Focusable"
+                Value="False" />
+        <Setter Property="Background"
+                Value="{DynamicResource BorderBrush}" />
+        <Setter Property="Margin"
+                Value="29,2,0,2" />
+        <Setter Property="Height"
+                Value="1" />
+        <Setter Property="Template">
+            <ControlTemplate>
+                <Border Background="{TemplateBinding Background}"
+                        BorderBrush="{TemplateBinding BorderBrush}"
+                        BorderThickness="{TemplateBinding BorderThickness}"
+                        CornerRadius="{TemplateBinding CornerRadius}" />
+            </ControlTemplate>
+        </Setter>
+    </ControlTheme>
+
+</ResourceDictionary>

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

@@ -1,5 +1,6 @@
 <ResourceDictionary xmlns="https://github.com/avaloniaui"
-                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
+                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+                    x:ClassModifier="internal">
 
     <ControlTheme x:Key="{x:Type TransitioningContentControl}"
                   TargetType="TransitioningContentControl">