Win11 Theming API proposals

[pchaurasia14]

Below are the first draft of API proposals for Win11 Theming -

• [API Proposal]: Win 11 Theming of Controls - Changes in Controls #8645
• [API Proposal]: Win11 Theming - Icon element support #8647
• Few more in the works (related to ThemeManager)

In this initial iteration (Iteration 0), our focus has been on incorporating the existing APIs from WPFUI as part of the preliminary integration effort. The API proposals outlined in the provided links have been derived from this effort.

We believe that few of these APIs would need refinements before we finalize them in the product.
Once we have an agreement on the final shape of the API in the issues, we will proceed with .NET-wide API approval process and subsequently have those implemented in code.

We welcome any suggestions and feedback on the design of these API to future proofing them and making them at par with other UI frameworks.

PS - Since we haven't had much XAML API reviews in the past, there is some learning curve to it on our side.

cc: @dotnet/dotnet-wpf-triage @pomianowski