feat: support multiple classes, improve unit tests.

tests/HeadlessTest.Ursa/Controls/DialogTests/StyleClassTests/TestWindow.axaml

@@ -3,6 +3,7 @@
         xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
         xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
         xmlns:u="https://irihi.tech/ursa"
+        xmlns:helpers="clr-namespace:Irihi.Avalonia.Shared.Helpers;assembly=Irihi.Avalonia.Shared"
         mc:Ignorable="d" d:DesignWidth="800"
         d:DesignHeight="450"
         x:Class="HeadlessTest.Ursa.Controls.DialogTests.StyleClassTests.TestWindow"
@@ -13,6 +14,11 @@
                 <Setter Property="Content" Value="CUSTOM"/>
             </Style>
         </Style>
+        <Style Selector="u|DefaultDialogControl.Custom2">
+            <Style Selector="^ /template/ Button#PART_OKButton">
+                <Setter Property="helpers:ClassHelper.Classes" Value="Warning Small" />
+            </Style>
+        </Style>
     </u:UrsaWindow.Styles>
     Welcome to Avalonia!