Added AvaloniaUI project.

Harmonia.UI/App.axaml

@@ -0,0 +1,18 @@
+<Application xmlns="https://github.com/avaloniaui"
+             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+			 xmlns:vm="clr-namespace:Harmonia.UI.ViewModels"
+             x:Class="Harmonia.UI.App"
+             RequestedThemeVariant="Default">
+             <!-- "Default" ThemeVariant follows system theme variant. "Dark" or "Light" are other available options. -->
+
+    <Application.Styles>
+        <FluentTheme />
+    </Application.Styles>
+
+	<Application.Resources>
+		<ResourceDictionary>
+			<vm:ViewModelLocator x:Key="Locator" />
+	    </ResourceDictionary>
+	</Application.Resources>
+
+</Application>