fix centering in plasma 6

[DaRandomCube]

after updating and installing the plasma 6 of the launcher, I noticed that the launcher isn't perfectly centered
here is what it looks like :

[slizgi]

I think it would be a nice option to have a simple switch in settings. One - it is centered relative to the screen width (win 11 style). The second - anchored with the position of the Launcher button.