ProgressBar: Pause indeterminate animation when invisible

Invisible items are not getting drawn, no matter how many times you call
their updateItem method. But to further improve the performance,
downstream styles like Breeze can hook into Item::visible state to stop
their internal animation engines from updating current value and even
attempting to redraw anything.

Also it still works fine with ShaderEffectSource: setting source item's
visibility affects whether or not it is displayed in ShaderEffectSource.
Hiding source item or not has no bearing on this matter.