plugins/slideback: Increase motion "strength" (inertia) to match global animation scale.

Since Plasma 6.0, the general unification of transition effects and properties fixed up most straggling effect animation durations (example for Windowheap durations). However, the Slide Back effect is "too quick" in relation to the global animation speed, this turns out to be the inertia rather than the speed/timing.

With this small adjustment to the strength/inertia calculation, it better matches the global Animation Scale up and down the scale (and matches 5.27). I've been using this as a patch since March this year and frankly forgot it was in my PKGBUILD.