Added motion event compression in xcb plugin.

Added motion event compression in xcb plugin.

We did this in 4.8 too, to avoid swamping the event queue with stale
motion events. If you're interested in fine grained motion events you
just need to have a responsive main loop. Most applications are only
interested in the latest mouse position in any case.

Fixes performance issues in QML where an onMouseXChanged or
onMouseYChanged in a mouse area does directly or indirectly runs some
heavy computations, slowing rendering to a halt since the main loop is
constantly busy.

Change-Id: I169c96458db4d57b689d6c2c915765b11c35e123
Reviewed-by: Jens Bache-Wiig <jens.bache-wiig@digia.com>