Fix property specifics for QtQuick3D.Repeater3D

The delegate property of Repeater3D should be Component type instead of Object3D, which doesn't work.

The model property is QVariant and supports many different types of input. However, all examples and tests in QtQuick3D only use it one way: to define the number of delegates to instantiate. Presumably the other ways are not very well tested/supported, so maybe it would make sense to change the control for model property to integer value from QVariant? And maybe in UI change it to show "Delegate Count" or something for clarity?