WheelHandler: Fix scrolling over ScrollBars on ScrollablePage

Code didn't handle re-parenting of Flickable to a ScrollView, which
occurs in ScrollablePage by design. This prevented scrolling over
scrollbars in such prominent places as list of System Settings modules
or content pane of Discover.

This solution handles re-parenting as well re-assigning either of the
two scrollbars via attached properties. The only precondition is that
ScrollBar attached object should exist before the target is set, so that
we have something to subscribe to.

BUG: 438526
(cherry picked from commit dba5d112)