We might want to different animations for different target value on an UiElement. For example, we might want to immediately show an element but gradually fade it out. The UI for showing speech recognition text result is such an example. We want text to appear immediately and fade out after showing up a few seconds.

Introduce the concept of target-value-specific duration would make this easier to implement as we can basically tell the UIElement to
1. If target value is visible, set 0 duration
2. If target value is invisible, set duration to a certain timeout
and then call element->SetVisible(value) normally.