add FocusVisualStyle for some controls

src/Shared/HandyControl_Shared/Themes/Styles/GridSplitter.xaml

@@ -0,0 +1,7 @@
+<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation">
+
+    <Style TargetType="GridSplitter">
+        <Setter Property="Focusable" Value="False"/>
+    </Style>
+
+</ResourceDictionary>