QtQuick.Dialogs Dialog: implicit geometry fixes

On platforms with in-scene dialogs (such as Android), implicitHeight
was updating too often, not at the optimal time, and not after
being set invisible.  The result was that if you added more buttons
while the dialog was invisible, it did not grow in height if needed.

Task-number: QTBUG-37285

Change-Id: I0b0ad230751361aed89b71b190f12e53d6c73a22
Reviewed-by: Liang Qi <liang.qi@digia.com>