[Samples] Proposal: Add samples that show collection item placeholders while loading data

[xperiandri]

Proposal: For each collection add samples that show collection item placeholders while loading data to XAML Controls Gallery and WinUI 3 Controls Gallery apps

Summary

Usually, when you don't have any data in your app immediately you want to show some placeholders (probably animated) that show the user that the data is being loaded.
Something similar to https://flutterappworld.com/flutter-listview-and-gridview-that-shows-loading-widgets/ or https://react-bootstrap.github.io/components/placeholder/

Rationale

• Makes an impression that the content is almost there
• Allows to keep user's attention longer
• There is no such sample available yet

Scope

Capability	Priority
This proposal will allow developers to quickly add placeholders to collection items	Must
This proposal will allow end-users to keep attention to the app longer	Must

[marcelwgn]

@gregwoo-microsoft Maybe this is more fitting for the XCG/WCG?

[gregwoo-microsoft]

Thanks for the suggestion @xperiandri . I agree that this could be a nice feature to add to the WinUI Gallery.

[karkarl]

This can be a new native control, or some managed implementation. There is no plans in the roadmap for this feature, but we welcome contributions.