System tray highlight animations are more consistent

I noticed some strange behavior in the animation of the system tray applet highlight.

For context, here's the system tray with the network applet active/highlighted:

Here's the weird behavior I was seeing, in both slow and default animation speeds:

old_tray_behavior.mkv

This is the problem: the indicator is bound to the active applet if there is one, otherwise it is bound to the tray, which makes sense. When an applet is closed, the indicator's parent changes from the applet to the entire tray. While the indicator is fading out, you can see it animate its x/y/width/height to match its new parent, the tray.

This MR change the behavior so in that case, we perform the opacity animation, but not the x/y/width/height animation.

new_tray_behavior.mkv

also, changed the Easing of existing animations from Quad to Cubic per the HIG