Add EditorConfig, fix formatting, remove unuses usings etc

[Saibamen]

1. Add EditorConfig from https://github.com/dotnet/aspnetcore repo
2. Sort and remove unused usings
3. var
4. Fix formatting
5. Remove trailing whitespaces
6. Add missing readonly
7. Avoid unnecessary assignment of value
8. Object initialization
9. Fix some private variable names
10. Remove unused GetFontSize
11. Remove redundant ToString() - this will also fix possible NREs
12. Curly braces to surround blocks of code
13. Convert to file-scoped namespace to reduce nesting
14. And more

Please hide spaces in diff for better CR experience:

[barnhill]

I can't review this since it touches almost every line

[Saibamen]

OK, I created https://github.com/barnhill/barcodelib/pull/194