A dataDevice has a request to start a drag with multiple parameters.
As kwayland's goal is to turn an event-driven API into a property cache API we
store this data. This patch moves that storage to the Seat as properties of the
active drag, rather than a property of the data device that happened to
initialise it.
This both helps keep a lot of other logic together, but also allows us to expose
a public startDrag method that can be invoked from Kwin's internal surfaces or
xwayland.
Any properties in DataDevice now relate to data being dropped on the device.
Every time the icon surface is committed, we need to accumulate the
surface offset. However, this is not easy to do on the compositor side
with the current design.
This change introduces a new class DragAndDropIcon that defines a
surface role as well as accumulates surface offsets on every commit.
This fixes a bug where the dnd icon jumps all of sudden while dragging
an image from Firefox to other applications.
