sidebar: Replace OpacityAnimator with NumberAnimation for proper fade in

Animator doesn't quite work in cases where you want to bind on its animated property value, such as here with opacity > 0. Setting id for animation object and binding on its running property does not always help either, as there may be multiple changes during single frame of an event loop, resulting in item getting invisible in the middle, which also prevents animator in its infinite lazyness from doing anything useful.