If you lift fingers but not swipe them enough to switch to another
virtual desktop, the slide effect will play an animation to move from
the current position in the virtual desktop grid to the current desktop.

However, that animation doesn't feel right, there's something missing.
The slide effect uses a TimeLine to animate switching between virtual
desktops, it's great if the amount of sliding is constant.

This change makes the slide effect use the mass-spring-damper model to
simulate the motion of a spring in order to animate switching between
virtual desktops.

The mass-spring-damper equation is integrated using RK4. If the delta
interval is not multiple of the integration step precisely, the
SpringMotion will perform integration as many times as the integration
step fits into the delta. The leftover will be used for LERP between the
previous and the next integration results.

With the spring animation, the slide animation feels more natural when
you lift fingers. If you switch between virtual desktops without using a
gesture, the slide animation should look almost the same as if it were
implemented with the TimeLine.