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.

(cherry picked from commit 1814f9f3)