DragItemsStartingEventArgs is sealed and doesn't provide public constructor

[hawkerm]

Describe the bug This is the most recent example I have, but all the EventArgs classes being sealed and not having constructors is a big problem when trying to implement custom controls which follow the same API interface as built-in controls.

I'd like to re-use the DragItemsStartingEventArgs to implement my own pair of DragItemsStarting/Completed events on a custom view control which allows its items to be dragged.

However, I can't use the same delegate and events as ListViewBase as the event args class DragItemsStartingEventArgs is sealed and therefore I can't set the properties on it as there's no constructor with parameters.

Expected behavior Able to use built-in event args classes and delegates to create controls with matching APIs to built-in controls.

Version Info WUX 19042 (but looks according to docs this is still the case in WinUI 3)

Windows 10 version	Saw the problem?
Insider Build (xxxxx)
October 2020 Update (19042)	Yes
May 2020 Update (19041)
November 2019 Update (18363)
May 2019 Update (18362)
October 2018 Update (17763)
April 2018 Update (17134)
Fall Creators Update (16299)
Creators Update (15063)

Device form factor	Saw the problem?
Desktop	Yes
Xbox
Surface Hub
IoT

Additional context Related to #780

[hawkerm]

I also can't as easily use the underlying DragStartingEventArgs and DropCompletedEvents as easily either, as OriginalSource is set only and I guess generally set by the ListViewBase class?

[StephenLPeters]

@MikeHillberg seems like a good candidate to unseal?

[hawkerm]

KeyRoutedEventArgs is the same (as pretty much every event args class).