Fix a slide-out loop when the mouse pointer is in the way

If the mouse pointer is sitting in the path of where the notification will slide off the screen, an enter-notify event gets triggered, which resets the notification's expiration and position back to the pre-slide-out spot.

Instead of resetting expiration on enter-notify, now we reset on motion-notify. If the pointer is sitting stationary next to the notification, and the notification moves under it, there won't be any motion-notify events.

A side-effect of this is that if a new notification appears under the mouse pointer, it will start expiring immediately. Previously, it would be 'held', since an enter-notify event would happen immediately after the notification appears. While this is a change in behavior, I'm not exactly convinced it's bad.

Closes #59 (closed).