[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.