Added nullable annotations to Avalonia.Controls.

[grokys]

What does the pull request do?

Adds nullable annotations to Avalonia.Controls and fixes resulting errors in Avalonia.Diagnostics.

This is a big one! I'm not sure I've got everything right here, so if you're responsible for a control/class here and the annotations look wrong to you, please simply push a fix to this branch if you have write access, otherwise comment and I'll fix.

[avaloniaui-team]

You can test this PR using the following package version. 0.10.999-cibuild0018327-beta. (feed url: https://nuget.avaloniaui.net/repository/avalonia-all/index.json) [PRBUILDID]

[maxkatz6]

if you're responsible for a control/class here and the annotations look wrong to you, please simply push a fix to this branch if you have write access

I am pretty sure we will come back to this after merging this PR anyway.

[maxkatz6]

Two sits review done. If I haven't missed anything (I probably have), it looks good.
Only some minor comments.

And merge conflicts are there.

[grokys]

@maxkatz6 you're a hero for wading through this! 🥇

Responded to your comments and fixed merge conflicts. We just need a quick bit of input from @kekekeks and @danwalmsley on a couple of those.

@Gillibald please let me know if the null handling of TextPresenter.Foreground makes sense.

[avaloniaui-team]

You can test this PR using the following package version. 0.10.999-cibuild0018413-beta. (feed url: https://nuget.avaloniaui.net/repository/avalonia-all/index.json) [PRBUILDID]