Move drag logic to seat and introduce SeatInterface::startDrag

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.