Dropzone: Reordered property to expose current order

[tesar-tech]

Closes #5830

360d019e-276b-461c-9b66-6a346552bd88.mp4

This function triggers every time the order changes:

• when an item is dragged to a new position
• when an item is added from a different zone
• when an item is removed

While removing and adding items isn’t technically reordering, I believe these actions should also trigger the function.

Currently, it also fires when a user drags an item and drops it back in the same position. While this doesn’t alter the collection, the function still invokes. We could compare the old and new dictionary to prevent this, but is it worth the additional complexity?

[mkontula]

@tesar-tech From the my perspective adding and removing items for example from zone "all options" to "selected and ordered options" is a reordering since you can drop the new item in any position in "selected and ordered options". Also when you remove an item from "selected and ordered options", you would expect the "selected and ordered options" internal and exposed order to be coherent and without gaps.

[stsrki]

Got some more ideas and added dropzone names to the DropZoneOrder as I think it will be useful to the users. Please review.