Winforms: Enable nullability across winforms

[hughbe]

Once CoreFx and CoreCLR fully support nullability it would be great to do this in winforms

Probably a large project...

[merriemcgaw]

Got a few things on the priority list, but we can consider adding this to the roadmap. @OliaG for Roadmap thoughts.

[Logerfo]

Implementing NRT to the whole code base would be a massive work, but enabling it only for the API surface would be much easier and yet very helpful for customers who uses both winforms and NRT.

[drewnoakes]

Something we did on project system, CPS and other projects is to add <Nullable>enable</Nullable> to project files, then use a regular expression to add #nullable disable to each and every source file.

This encourages new code to be written using NRT, and provides an annotation to-do list by searching for #nullable disable.

For commonly used types where annotations would be useful, but it's too challenging to fix all generated warnings, it's possible to use #nullable annotations which allows adding ? to types without having to address any warnings.

[RussKie]

Thank you @drewnoakes, this may be a way forward.

[Logerfo]

```c#

nullable annotations

```
@drewnoakes where is this documented? VS syntax highlight it, but the compiler is bothered:

CS8637 Expected 'enable', 'disable', or 'restore'

Edit: Oh, nevermind, got it. It's #nullable enable annotations.

[sharwell]

I've done this for a few projects to date, and may be able to help here.

[Logerfo]

Now that #2702 and #2756 are merged, I encourage everyone interested to send PRs covering small parts of the API for NRT in SWF.

[RussKie]

When annotating Windows Forms types verify that the Runtime types are NRT enabled (refer to https://github.com/dotnet/runtime/issues/2339).