feat: implement inner content interface.

demo/Ursa.Demo/Pages/MultiAutoCompleteBoxDemo.axaml

@@ -7,9 +7,11 @@
              mc:Ignorable="d" d:DesignWidth="800" d:DesignHeight="450"
              x:DataType="vm:MultiAutoCompleteBoxDemoViewModel"
              x:Class="Ursa.Demo.Pages.MultiAutoCompleteBoxDemo">
-    <StackPanel Spacing="20">
+    <StackPanel Spacing="20" HorizontalAlignment="Left">
         <TextBlock Text="Multi-AutoCompleteBox"/>
         <u:MultiAutoCompleteBox ItemsSource="{Binding Items}" 
+                                MaxWidth="400"
+                                InnerLeftContent="Controls"
                                 SelectedItems="{Binding SelectedItems}"
                                 ItemFilter="{Binding FilterPredicate }"
                                 FilterMode="Custom">

demo/Ursa.Demo/ViewModels/MultiAutoCompleteBoxDemoViewModel.cs

@@ -1,4 +1,5 @@
-using System.Collections.Generic;
+using System;
+using System.Collections.Generic;
 using System.Collections.ObjectModel;
 using Avalonia.Controls;
 using CommunityToolkit.Mvvm.ComponentModel;
@@ -82,12 +83,13 @@ public class MultiAutoCompleteBoxDemoViewModel: ObservableObject
     {
         if (text is null) return true;
         if (data is not ControlData control) return false;
-        return control.MenuHeader.Contains(text )|| control.Chinese.Contains(text);
+        return control.MenuHeader.Contains(text, StringComparison.InvariantCultureIgnoreCase) ||
+               control.Chinese.Contains(text, StringComparison.InvariantCultureIgnoreCase);
     }
 }
 
 public class ControlData
 {
-    public string MenuHeader { get; set; }
-    public string Chinese { get; set; }
+    public required string MenuHeader { get; init; }
+    public required string Chinese { get; init; }
 }