Add drag-drop item reordering to CollectionView #3767

[billvenhaus]

Description of Change

Implements drag-drop item reordering for CollectionView #3767

Additions made

• Changes base class of CollectionView to ReorderableItemsView.
• Adds CanReorderItems property to ReorderableItemsView.
• Adds CanMixGroups property to ReorderableItemsView.
• Adds ReorderCompleted event to ReorderableItemsView.
• Adds Reordering Galleries to CollectionView Gallery

Pictures

PR Checklist

• Targets the correct branch
• Tests are passing (or failures are unrelated)
• Targets a single property for a single control (or intertwined few properties)
• Adds the property to the appropriate interface
• Avoids any changes not essential to the handler property
• Adds the mapping to the PropertyMapper in the handler
• Adds the mapping method to the WinUI, Android, iOS, and Standard aspects of the handler
• Implements the actual property updates (usually in extension methods in the Platform section of Core)
• Tags ported renderer methods with [PortHandler]
• Adds an example of the property to the sample project (MainPage)
• Adds the property to the stub class
• Implements basic property tests in DeviceTests

Does this PR touch anything that might affect accessibility?

• Does this PR introduce a new control? (If yes, add an example using SemanticProperties to the SemanticsPage)
• APIs that modify focusability?
• APIs that modify any text property on a control?
• Does this PR modify view nesting or view arrangement in anyway?
• Is there the smallest possibility that your PR will change accessibility?
• I'm not sure, please help me

If any of the above checkboxes apply to your PR, then the PR will need to provide testing to demonstrate that accessibility still works.

[pictos]

Can you label those ints in the ValueTuple? To be more explicit on what those means, like (int x, int y)

[billvenhaus]

Sure! I just committed the change.

[hartez]

/azp run

[azure-pipelines]

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