Add easing curve to Morphing Popups

I noticed that the Morphing Popups effect (which is enabled by default) doesn't have an easing curve, which makes such animations look more unnatural and less responsive than most other built-in effects. This MR aims to solve that by adding an in-out cubic curve to the geometry change animation.

I'm not sure if it's necessary to add a curve to the crossfade part though. For example, the Fade Desktop effect for switching between virtual desktops uses a linear curve (the same as no curve parameter in animate()) for both fading in and out, and the Fade effect for windows uses OutQuart only when the window is closed.