Investigate potential of using Avalonia in place of WPF

This is definitely more of a long term goal and may never go anywhere, but it's useful to keep track of it

http://avaloniaui.net is a cross platform UI framework inspired by WPF - potentially we could change to using it in place of WPF or use project configuration to target both frameworks in different builds.

Keep an eye on Avalonia and see how it gets on, plus what it's performance and future maintenance look like