fix: 1. simplify code for nullable control event assignment.

2. make textbox readonly and dragpanel visible controlled in code behind.
3. Use compile binding in demo.

demo/Ursa.Demo/Pages/NumericUpDownDemo.axaml

@@ -5,6 +5,9 @@
     xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
     xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
     xmlns:u="https://irihi.tech/ursa"
+    xmlns:vm="using:Ursa.Demo.ViewModels"
+    x:DataType="vm:NumericUpDownDemoViewModel"
+    x:CompileBindings="True"
     d:DesignHeight="450"
     d:DesignWidth="800"
     mc:Ignorable="d">