data_device: get rid of attach request

In the effort to make everything a regular surface, remove
data_device.attach request. To maintan the functionality, add
an icon surface parameter to data_device.start_drag.

Signed-off-by: Kristian Høgsberg <krh@bitplanet.net>
Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>