feat: add option to make it selectable.

demo/Ursa.Demo/Pages/NumberDisplayerDemo.axaml

@@ -19,5 +19,13 @@
             FontSize="30"
             StringFormat="yyyy-MM-dd"
             Value="{Binding DateValue}" />
+        <TextBlock Text="Selectable: "></TextBlock>
+        <u:Int32Displayer Value="{Binding Value}" IsSelectable="True" />
+        <u:DoubleDisplayer StringFormat="N2" Value="{Binding DoubleValue}" IsSelectable="True" />
+        <u:DateDisplay
+            FontSize="30"
+            StringFormat="yyyy-MM-dd"
+            IsSelectable="True"
+            Value="{Binding DateValue}" />
     </StackPanel>
 </UserControl>

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

@@ -16,9 +16,23 @@
                     FontWeight="{TemplateBinding FontWeight}"
                     FontStyle="{TemplateBinding FontSize}"
                     FontStretch="{TemplateBinding FontStretch}"
-                    Text="{TemplateBinding InternalText,
-                                           Mode=OneWay}" />
+                    Text="{TemplateBinding InternalText, Mode=OneWay}" />
             </ControlTemplate>
         </Setter>
+        <Style Selector="^[IsSelectable=True]">
+            <Setter Property="Template">
+                <ControlTemplate TargetType="u:NumberDisplayerBase">
+                    <SelectableTextBlock
+                        FontSize="{TemplateBinding FontSize}"
+                        Foreground="{TemplateBinding Foreground}"
+                        FontFamily="{TemplateBinding FontFamily}"
+                        Background="{TemplateBinding Background}"
+                        FontWeight="{TemplateBinding FontWeight}"
+                        FontStyle="{TemplateBinding FontSize}"
+                        FontStretch="{TemplateBinding FontStretch}"
+                        Text="{TemplateBinding InternalText, Mode=OneWay}" />
+                </ControlTemplate>
+            </Setter>
+        </Style>
     </ControlTheme>
 </ResourceDictionary>

src/Ursa/Controls/NumberDisplayer/NumberDisplayerBase.cs

@@ -35,6 +35,15 @@ public abstract class NumberDisplayerBase : TemplatedControl
         get => GetValue(StringFormatProperty);
         set => SetValue(StringFormatProperty, value);
     }
+
+    public static readonly StyledProperty<bool> IsSelectableProperty = AvaloniaProperty.Register<NumberDisplayerBase, bool>(
+        nameof(IsSelectable));
+
+    public bool IsSelectable
+    {
+        get => GetValue(IsSelectableProperty);
+        set => SetValue(IsSelectableProperty, value);
+    }
 }
 
 public abstract class NumberDisplayer<T>: NumberDisplayerBase