Ensure ErrorProvider supports all accessibility requirements.

[merriemcgaw]

Per: https://docs.microsoft.com/en-us/dotnet/framework/winforms/controls/errorprovider-component-overview-windows-forms The ErrorProvider doesn't support accessibility clients. We should add that because apps that depend on it may not be fully accessible for their users.

[RussKie]

This may be handy as well https://blogs.msdn.microsoft.com/winuiautomation/2017/09/14/let-your-customers-know-of-important-status-changes-in-your-winforms-app/

[vladimir-krestov]

@merriemcgaw, @RussKie, @Tanya-Solyanik, could you please say if I got it correctly?
I have implemented UIA supporting for ErrorProvider.ErrorWindow and ErrorProvider.ErrorWindowAccessibleObject.
Now we can change ErrorProvider AccessibleObject properties. But I couldn't find any same implementation, so I need your suggestion for how we'll implement ErrorProvider AccessibleObject properties.
At the moment, I have implemented it like this:

I found the following bug:

• There is one error icon
  

• There are two error icons
  

Both icons have one ErrorWindow and one AccessibleObject.
It isn't a regression, it reproduces in the release branch.
Do we need to create a separated bug to fix it?

Now Narrator can announce an error icon AccessibleObject if a user uses CapsLock + keyboard arrows:

[vladimir-krestov]

Quick fix. I can make like this:

[vladimir-krestov]

When hovering by mouse you get icon AccessibleObject:

Narrator works well using CapsLock + keyboard arrows combination:

@merriemcgaw, @Tanya-Solyanik, @RussKie, could you please confirm my solution?

[vladimir-krestov]

For details please see #2381.