Starts a drag and creates a new drag context for it.
This function is called by the drag source.
| window |
the source window for this drag |
| device |
the device that controls this drag |
| targets |
the offered targets, as list of Atoms |
| x_root |
the x coordinate where the drag nominally started |
| y_root |
the y coordinate where the drag nominally started |
|
a newly created DragContext |