ScrollBar: Fix size being initially stuck on Qt 6

Due to a bug in Qt, explicit binding to ScrollBar::size is required to help it notify the binding to reevaluate. According to my and Noah's investigation, size can indeed change in a way that affects getter of visualSize, but only signal sizeChanged will be dispatched, thus leaving visualSize hanging in a limbo state when querying it manually would result in a new value but change listeners would never know that.

BUG: 473241
See also: https://codereview.qt-project.org/c/qt/qtdeclarative/+/501319