[SliderGroove] Rework sizing

x, y, width and height are now manually set. There is a bit of extra
space around the groove than before.

Fill uses anchors and anchor margins to set size and position, which
makes layout mirroring easier.

Set velocity to 800 explicitly

Add tickmark code, but keep it commented out. Yes, I shouldn't include
commented out code, but I really don't feel like keeping it in a branch.
It works, but I'm not sure if KDE app devs want it or not.

(cherry picked from commit cf3c7f15)