[WIP] Relax a few files to welcome a port to Uno Platform

[lextm]

The goal is to follow up the same approach my team applied during Project Rover development,

• Split AvalonEdit files to platform neutral .cs and WPF specific .wpf.cs files.
• Write shims and Uno specific implementation in .uno.cs files.
• Combine the neutral and Uno-specific files with a complete Uno port, currently named UnoEdit for simplicity.

The prototype is ready and will be made public soon.

This pull request will be further updated as we progress.

Going to resolve #248 and #290.

[lextm]

With the upcoming open source cross-platform WPF fork, the Uno Platform port will stop. Close this now.

[jogibear9988]

@lextm wich fork??

[lextm]

@jogibear9988 https://github.com/wieslawsoltes/wpf/tree/progpu-rendering-port The entire code base (and ProGPU) is now open-sourced.