feat: WIP add a demo.

demo/Ursa.Demo/Pages/MultiComboBoxDemo.axaml

@@ -0,0 +1,14 @@
+<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"
+             mc:Ignorable="d" d:DesignWidth="800" d:DesignHeight="450"
+             xmlns:vm="using:Ursa.Demo.ViewModels"
+             xmlns:u="https://irihi.tech/ursa"
+             x:DataType="vm:MultiComboBoxDemoViewModel"
+             x:CompileBindings="True"
+             x:Class="Ursa.Demo.Pages.MultiComboBoxDemo">
+    <StackPanel>
+        <u:MultiComboBox ItemsSource="{Binding Items}"/>
+    </StackPanel>
+</UserControl>