Proposal: Improve Microsoft.UI.Xaml.Window by Adding Dependency Property Support

The current **Microsoft.UI.Xaml.Window** class in WinUI lacks convenience APIs such as Width, Height, and other essential properties that developers commonly use in XAML-based applications. Additionally, **Window** does not inherit from DependencyObject or UIElement, preventing developers from adding **DependencyProperty** to it. While simple or full properties can be added, they do not function as expected in XAML at runtime and only work once when the application launches. Given that these limitations have persisted for years, it is crucial to support for DependencyProperty to allow more flexibility in windowing scenarios.

After a few years, the convenience api for window were finally unveiled in v1.7-exp3, but everyone was disappointed. Because access to it was allowed through the **AppWindow**.

Therefore, it is better to provide a way for creating dependency property. So that developers can easily meet their own needs and not wait months for Disappointing features from Microsoft.

Start by reviewing Microsoft.UI.Xaml.Window and the existing AppWindow path, then compare how DependencyObject and DependencyProperty are exposed elsewhere in WinUI. The issue names no implementation files or tests; completion would require a defined, tested design for dependency-property support and the requested window properties.

Written by the indexing model from the issue text.