Add Avalonia topic

Author: worldbeater

topics/avalonia/avalonia.png

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+---
+created_by: Steven Kirk
+display_name: Avalonia
+github_url: https://github.com/avaloniaui/avalonia
+logo: avalonia.png
+released: 2016
+short_description: A cross-platform XAML UI framework for .NET Core, .NET Framework and Mono
+topic: avalonia
+url: http://avaloniaui.net
+---
+Avalonia supports Windows, Linux and OSX with experimental mobile support for Android and iOS. Avalonia uses a XAML dialect that should feel immediately familiar to anyone coming from WPF, UWP and Xamarin Forms.