Draft: applets/pager: Use NumberAnimation for opacity animation
The animation driver is shared among windows, unless a QQuickRenderControl object is created for a particular window. In addition to that, the QSGThreadedRenderLoop doesn't have a notion of window affinity for animations.
So, if there are two windows and one of them is hidden and has an item whose opacity is animated using OpacityAnimator, then the threaded render loop will force repaints for the other window even though it has no animations or nothing has updated in it. It is a design flaw in QtQuick.
To remedy that issue, this change ports the pager to NumberAnimation for the opacity animation.