feat: working on selection. add a demo to AutoCompleteBoxDemo to show the keyboard navigation actually changed selection.

src/Ursa.Themes.Semi/Controls/MultiAutoCompleteBox.axaml

@@ -24,7 +24,7 @@
                                     <u:ClosableTag 
                                         Content="{Binding .}" 
                                         Command="{Binding $parent[u:TagInput].Close}"
-                                        ContentTemplate="{TemplateBinding SelectedItemTemplate}" 
+                                        ContentTemplate="{Binding $parent[u:MultiAutoCompleteBox].SelectedItemTemplate}" 
                                         ></u:ClosableTag>
                                 </DataTemplate>
                             </ItemsControl.ItemTemplate>

src/Ursa/Controls/AutoCompleteBox/MultiAutoCompleteBox.cs

@@ -1630,6 +1630,8 @@ public partial class MultiAutoCompleteBox : TemplatedControl, IInnerContentContr
     {
         // TODO set selection.
         // SetCurrentValue(SelectedItemProperty, _adapter!.SelectedItem);
+        SelectedItems?.Add(_adapter?.SelectedItem);
+        // UpdateTextValue(null);
     }
 
     //TODO Check UpdateTextCompletion