Don't emit moving and flicking signals unnecessarily

Don't emit moving and flicking signals unnecessarily

The moving and flicking signals should only be emitted once when
the view has been moved/flicked both vertically and horizontally. (This
was already done correctly for the dragging signals.)

Also changes QQuickFlickable::flick() to return bool instead of void.
Subclasses no longer emit the flicking signals but call
flickingStarted() instead.

Also splits the tst_qquickflickable::movingAndDragging() test up into
several tests.

Change-Id: Ie527568a9702049dd0bcda18c2eb3e43d8938a18
Reviewed-by: Martin Jones <martin.jones@nokia.com>