Fix item polishing

After "Dont call updatePolish if an item is not visible" (439f31f1) and
"Fix polishItems bug" (01e609e9

), desktop style animations for QtQuick
Controls were no longer running. This was because QQuickItem::polish()
no longer polished a _visible_ item when there were other _hidden_
items in the queue to be polished.

This change restores the old logic that QQuickWindow only keeps track
of visible items to be polished, whilst the idea of hidden items not
being polished still remains valid. QQuickItem is made responsible
for polishing itself if necessary when it becomes visible.

Task-number: QTBUG-36934
Change-Id: I4d48d3a3e2c841d337cd52ec4fd27092f84a8626
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>