Moved to .NET Core, NuGet Packaging, and SFML.Net 2.5

Works cross-platform with the move to .NET Core. Tested on Linux and Windows.
All examples working again as some seemed to have been in a broken state.
Added project files for each example, targeting netcoreapp3.0.
Added .editorconfig file to help aid other contributors to keep the same coding style.
'Broke out' enums and classes into separate .cs files if there were within another.
General code cleanup.

This pull request addresses #1 and #3

kylepulver / Otter