Proposal: Support Window Width/Height when in Desktop

Summary

For WinUI in Desktop apps:

• Support setting Width and Height on Window in XAML.
• Support setting d:DesignWidth and d:DesignHeight on Window in XAML.

<Window Width="1200" Height="600"
        d:DesignWidth="1000" d:DesignHeight="500">

Rationale

Many developers doing WinUI in Desktop will have a background in WPF and/or simply not be interested in doing a responsive design -- this is "in Desktop" after all. Without the ability to set Window size in a traditional way, I believe adoption will suffer due to too much forced change.