Don't deliver drag move events immediately.

Don't deliver drag move events immediately.

Rather than delivering drag move events everytime the position of an
item changes queue up an event to process in the event loop.  This
filters out noisy intermediate positions particularly where the x and
y values are set independently and better insulates against feedback.

Change-Id: I5d787d63ed01441a9080d0daaee9db1373d5f073
Reviewed-by: Martin Jones <martin.jones@nokia.com>