Desktop style animations are not running

After the following changes in qtdeclarative:

• Change Idad6107a: Dont call updatePolish if an item is not visible
  https://codereview.qt-project.org/#change,73841

• Change I1ad929dc: Fix polishItems bug
  https://codereview.qt-project.org/#change,76857

the desktop style animations (eg. indeterminate progress bar) are not running at all. QQuickItem::polish() doesn't call QQuickWindow::maybeUpdate() when appropriate anymore, because QQuickWindowPrivate::itemsToPolish now contains hidden items.

Steps to reproduce:

• run qtquick controls gallery
• the progress bar is not animating
• hover busy indicator (below slider) => animations start running