Add easing to floating panel animation

The floating panel has been recently merged but there's no easing curve to its animation. I'm using Easing.OutCubic since it's consistent with other effects, like Maximize/Fullscreen or the Overview effect. The difference is subtle, but the animation feels snappier than before. @niccolove @ngraham

Before - Slow Speed	This MR - Slow Speed
no_easing_slow	out_cubic_slow

Before - Normal Speed	This MR - Normal Speed
no_easing_normal	out_cubic_normal