Data device interface in client just handle with pointer's dnd.
If a touch screen trigger dnd, it will use pointer struct like i
nput->sx, input->sy, input->pointer_focus. So if pointer is moving
when touch screen trigeer a dnd, wrong behaviore will occur.
Before touch screen start dnd, system call touch_grab()
to mark the following drag and drop operation is generated by
touch screen.
Defined some common variables in struct input to track dnd.

Note, touch screen and pointer can't generate drag and drop at the
same time, becuae data device protocol can't identify the drag
and drop event is generated by touch screen or pointer.

Signed-off-by: Xiong Zhang <xiong.y.zhang@intel.com>
Reviewed-by: Kristian Hogsberg <hoegsberg@gmail.com>