Use "Centered" window placement by default

The current "Minimize Overlapping" window placement tends to position windows in locations that seem completely random, typically in a screen corner. It is doing this because, true to its name, it is trying to avoid overlapping other windows as much as possible. However in practice this is rarely helpful. When the user opens a new window, it's because they want to use it, and positioning the window far from where the user is likely to be looking is counter-productive. This is even more true on today's large and wide displays, where placing the window in a corner may position it entirely outside the user's current field of vision. We get bug reports about this exact issue for notifications (which always appear in a corner by default) by users of such screens. For notifications, this can be justifiable because notifications are designed to be ignorable; app windows on the other hand, are not.

As a result, I commonly see Plasma users open windows and then immediately, reflexively grab the window's titlebar and drag it to the center of the screen. I have seen my wife do this. I have seen every YouTube reviewer of Plasma do this. I have even see fellow KDE developers at sprints do this. It seems like quite a common impulse to want a newly-opened window to appear in the center of the screen, which is where the user is likely to already be looking.

Thankfully, KWin already has a window placement mode that does this automatically: "Centered". Accordingly, this commit changes the default KWin window placement mode from "Minimize Overlapping" to "Centered".

No kconf migration script is provided because this is a better default for most people in most cases, and existing users are highly likely to appreciate this change.

cc @teams/usability cc @teams/vdg