Change the device added event to a synchronous new_device callback

In order to allow the client to configure the initial values of a
discovered device before added to the pool, change the device added
event into a synchronous new_device callback.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>
Remove get_current_screen_dimension callback

Replace it with device configuration API.

Signed-off-by: Jonas Ådahl <jadahl@gmail.com>