feat: 1. update container state from selection collection change.

2. Add popup slot.
3. fix various binding relative resource issue.
4. update empty pseudo-class handing, simplify watermark visibility.

demo/Ursa.Demo/ViewModels/MultiComboBoxDemoViewModel.cs

@@ -1,12 +1,44 @@
-using System.Collections.ObjectModel;
+using System.Collections.Generic;
+using System.Collections.ObjectModel;
+using System.Windows.Input;
 using CommunityToolkit.Mvvm.ComponentModel;
+using CommunityToolkit.Mvvm.Input;
 
 namespace Ursa.Demo.ViewModels;
 
 public class MultiComboBoxDemoViewModel: ObservableObject
 {
     public ObservableCollection<string> Items { get; set; }
+    
+    public ObservableCollection<string> SelectedItems { get; set; }
 
+    public ICommand SelectAllCommand => new RelayCommand(() =>
+    {
+        SelectedItems.Clear();
+        foreach (var item in Items)
+        {
+            SelectedItems.Add(item);
+        }
+    });
+    
+    public ICommand ClearAllCommand => new RelayCommand(() =>
+    {
+        SelectedItems.Clear();
+    });
+    
+    public ICommand InvertSelectionCommand => new RelayCommand(() =>
+    {
+        var selectedItems = new List<string>(SelectedItems);
+        SelectedItems.Clear();
+        foreach (var item in Items)
+        {
+            if (!selectedItems.Contains(item))
+            {
+                SelectedItems.Add(item);
+            }
+        }
+    });
+    
     public MultiComboBoxDemoViewModel()
     {
         Items = new ObservableCollection<string>()
@@ -47,5 +79,6 @@ public class MultiComboBoxDemoViewModel: ObservableObject
             "Pennsylvania",
             "Rhode Island",
         };
+        SelectedItems = new ObservableCollection<string>();
     }
 }