feat: add selection list.

demo/Ursa.Demo/Pages/SelectionBoxDemo.axaml

@@ -0,0 +1,15 @@
+<UserControl xmlns="https://github.com/avaloniaui"
+             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+             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:SelectionListDemoViewModel"
+             x:CompileBindings="True"
+             mc:Ignorable="d" d:DesignWidth="800"
+             d:DesignHeight="450"
+             x:Class="Ursa.Demo.Pages.SelectionListDemo">
+    <StackPanel HorizontalAlignment="Left">
+        <u:SelectionList ItemsSource="{Binding Items}"></u:SelectionList>
+    </StackPanel>
+</UserControl>