Enhance DND protocol to be able to send scroll events to surface under

This is a follow-up from https://gitlab.gnome.org/GNOME/gtk/-/issues/314 which stated the fix must first be done in wayland-protocols.

When dragging an item, MacOS and Windows have the ability to scroll the target before dropping, that is very useful in some cases, eg. In a file manager app you drag a file and want to drop it over a folder to move it into, but that folder is out of view and you need to scroll to reveal it. This is done by moving the mouse scrollwheel while still holding the dragged file over your target widget.

In GNOME that feature worked for a small time in Nautilus, but stopped working with the migration to Wayland.

It would be nice to be on par with other modern desktops and allow to perform this useful mouse scrolling while dragging.