Added AvaloniaUI project.

Harmonia.UI/Views/MainWindow.axaml

@@ -0,0 +1,12 @@
+<Window xmlns="https://github.com/avaloniaui"
+        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+        xmlns:vm="using:Harmonia.UI.ViewModels"
+        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
+        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
+        xmlns:views="clr-namespace:Harmonia.UI.Views"
+        mc:Ignorable="d" d:DesignWidth="800" d:DesignHeight="450"
+        x:Class="Harmonia.UI.Views.MainWindow"
+        Icon="/Assets/avalonia-logo.ico"
+        Title="Harmonia.UI">
+        <views:MainView />
+</Window>