feat: MultiComboBox may support placeholder.

demo/Ursa.Demo/Pages/MultiComboBoxDemo.axaml

@@ -14,6 +14,7 @@
     <StackPanel Orientation="Horizontal">
         <u:MultiComboBox
             Name="combo"
+            PlaceholderText="Please Select"
             InnerLeftContent="Left"
             InnerRightContent="Right"
             Classes="ClearButton"

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

@@ -42,6 +42,18 @@
                                     Foreground="{DynamicResource TextBoxInnerForeground}"
                                     IsVisible="{TemplateBinding InnerLeftContent,
                                                                 Converter={x:Static ObjectConverters.IsNotNull}}" />
+                                <TextBlock
+                                  x:Name="PlaceholderTextBlock"
+                                  Grid.Column="1"
+                                  Grid.ColumnSpan="2"
+                                  Margin="{TemplateBinding Padding}"
+                                  TextTrimming="CharacterEllipsis"
+                                  HorizontalAlignment="Left"
+                                  VerticalAlignment="Center"
+                                  Foreground="{TemplateBinding Foreground}"
+                                  IsVisible="False"
+                                  Opacity="0.3"
+                                  Text="{TemplateBinding PlaceholderText}" />
                                 <ScrollViewer
                                     Grid.Column="1"
                                     Grid.ColumnSpan="2"
@@ -135,6 +147,9 @@
             </Style>
         </Style>
 
+        <Style Selector="^:selection-empty /template/ TextBlock#PlaceholderTextBlock">
+            <Setter Property="IsVisible" Value="{Binding $parent[ComboBox].SelectionBoxItem, Converter={x:Static ObjectConverters.IsNotNull}}" />
+        </Style>
 
         <!--  Pointerover State  -->
         <Style Selector="^:pointerover">

src/Ursa/Controls/ComboBox/MultiComboBox.cs

@@ -87,7 +87,16 @@ public class MultiComboBox: SelectingItemsControl, IInnerContentControl
         get => GetValue(SelectedItemTemplateProperty);
         set => SetValue(SelectedItemTemplateProperty, value);
     }
-    
+
+    public static readonly StyledProperty<string?> PlaceholderTextProperty =
+        ComboBox.PlaceholderTextProperty.AddOwner<MultiComboBox>();
+
+    public string? PlaceholderText
+    {
+        get => GetValue(PlaceholderTextProperty);
+        set => SetValue(PlaceholderTextProperty, value);
+    }
+
     static MultiComboBox()
     {
         FocusableProperty.OverrideDefaultValue<MultiComboBox>(true);