Proposal: Unseal all the things

Summary

Rationale

• Enable developers to subclass a wider range of our classes/controls (longstanding request from our community)

Scope

Capability	Priority
Unseal all, or at least nearly all classes in the framework	Must

Important Notes

• To WinUI community: Internally our MSFT XAML team has been planning to do this. I'm opening this issue to encourage additional conversation / interaction / questions about this work.
• This work will require WinUI 3, so I've added the needs-winui-3 label to the issue.