Added the ability to copy from a readonly textbox

samples/ControlCatalog/Pages/TextBoxPage.xaml

@@ -9,7 +9,7 @@
               Gap="16">
       <StackPanel Orientation="Vertical" Gap="8">
         <TextBox Text="Lorem ipsum dolor sit amet, consectetur adipiscing elit." Width="200" />
-        <TextBox Watermark="ReadOnly" IsReadOnly="True"/>
+        <TextBox Watermark="ReadOnly" IsReadOnly="True" Text="This is read only"/>
         <TextBox Width="200" Watermark="Watermark" />
         <TextBox Width="200"
                  Watermark="Floating Watermark"

src/Avalonia.Controls/Presenters/TextPresenter.cs

@@ -189,6 +189,12 @@ namespace Avalonia.Controls.Presenters
                     _caretTimer.Start();
                     InvalidateVisual();
                 }
+                else
+                {
+                    _caretTimer.Start();
+                    InvalidateVisual();
+                    _caretTimer.Stop();
+                }
 
                 if (IsMeasureValid)
                 {