feat: add avalonia TransitioningContentControl style.

Author: NaBian

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

@@ -18,6 +18,7 @@
                 <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/TransitioningContentControl.axaml" />
                 <MergeResourceInclude Source="avares://HandyControl/Themes/Styles/PopupRoot.axaml" />
                 <MergeResourceInclude Source="avares://HandyControl/Themes/Styles/PathIcon.axaml" />
                 <MergeResourceInclude Source="avares://HandyControl/Themes/Styles/ToolTip.axaml" />

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

@@ -0,0 +1,32 @@
+<ResourceDictionary xmlns="https://github.com/avaloniaui"
+                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
+
+    <ControlTheme x:Key="{x:Type TransitioningContentControl}"
+                  TargetType="TransitioningContentControl">
+        <Setter Property="Template">
+            <ControlTemplate>
+                <Panel>
+                    <ContentPresenter Name="PART_ContentPresenter"
+                                      Background="{TemplateBinding Background}"
+                                      BorderBrush="{TemplateBinding BorderBrush}"
+                                      BorderThickness="{TemplateBinding BorderThickness}"
+                                      CornerRadius="{TemplateBinding CornerRadius}"
+                                      ContentTemplate="{TemplateBinding ContentTemplate}"
+                                      Padding="{TemplateBinding Padding}"
+                                      VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}"
+                                      HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}" />
+                    <ContentPresenter Name="PART_ContentPresenter2"
+                                      Background="{TemplateBinding Background}"
+                                      BorderBrush="{TemplateBinding BorderBrush}"
+                                      BorderThickness="{TemplateBinding BorderThickness}"
+                                      CornerRadius="{TemplateBinding CornerRadius}"
+                                      ContentTemplate="{TemplateBinding ContentTemplate}"
+                                      Padding="{TemplateBinding Padding}"
+                                      VerticalContentAlignment="{TemplateBinding VerticalContentAlignment}"
+                                      HorizontalContentAlignment="{TemplateBinding HorizontalContentAlignment}" />
+                </Panel>
+            </ControlTemplate>
+        </Setter>
+    </ControlTheme>
+
+</ResourceDictionary>