Implement IsAncestorSiteInDesignMode.

[KlausLoeffelmann]

Fixes #4146.
Resolves #3300

[Update]: This is now implemented as originally planned.

Wrote a small Test app for this, which should also serve as a guidance for best practice.
It is currently here: https://github.com/KlausLoeffelmann/IsAncestorSiteInDesignMode/blob/main/README.md

Shows where and how to use correctly DesignMode and IsAncestorSiteInDesignMode for (derived) Forms, UserControls hosted in Forms and Controls:

Microsoft Reviewers: Open in CodeFlow

[merriemcgaw]

I'm very much OK with going forward with the original proposed API as approved.

[KlausLoeffelmann]

Yes. I had a discussion about this today with @JeremyKuhne, and he said the same thing.
If we want more options, we do that with additional APIs later. Also, I think we should wait for the License discussion to be resolved.

[merriemcgaw]

Let's not block on licensing for this effort.

[KlausLoeffelmann]

No, I mean with a potential additional API. This should fly as it is now.

[RussKie]

@KlausLoeffelmann could you please clarify the reason why we didn't add this API to Component class, where DesignMode property is defined? So we can document it.

[KlausLoeffelmann]

A Component is not part of the control's collection. It doesn't work that way. You could not traverse up to check a parent for being sited, since there isn't one.