Feature: Allow cancelling the ContentDialog by klicking into the background

[gentledepp]

On Mobile platforms, people are used to be able to cancel a dialog by clicking next to it (i.e. into the background)

It would be nice, if there was an event BackgroundClicked on the ContentDialog that is fired, whenever the BackgroundElement is clicked. (So it'd needed to be renamed to PART_BackgroundElement of course.

Would that be acceptable?

If so, i would volunteer to create a PR for this if you like :-)