plugins/slideback: Increase motion strength to match global animation scale

Since Plasma 6.0, the general unification of transition effects and
properties fixed up most straggling effect animation 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.

src/plugins/slideback/motionmanager.cpp

@@ -68,11 +68,11 @@ void WindowMotionManager::manage(EffectWindow *w)
         return;
     }
 
-    double strength = 0.08;
+    double strength = 0.12;
     double smoothness = 4.0;
     if (m_useGlobalAnimationModifier && effects->animationTimeFactor()) {
         // If the factor is == 0 then we just skip the calculation completely
-        strength = 0.08 / effects->animationTimeFactor();
+        strength = 0.12 / effects->animationTimeFactor();
         smoothness = effects->animationTimeFactor() * 4.0;
     }