qml: fix multiple rendering of the main view

... by making FrostedGlassEffect to be used as layer effect.

Currently, every time a FrostedGlassEffect is used, the source item needs to be rerendered. By using this effect in the source as a layer effect, the source item with its whole hierarchy is rendered one time offscreen, then the effect takes care of displaying both the effect and the source itself (effect is only applied to the area denoted by effectRect).