Currently in git master there are three options for Tray icon sizing/arrangement:
1. Scale to fit + automatic number of rows/columns
2. One auto-scaling row/column
3. Two auto-scaling rows/columns

After consultation with VDG folks and many discussions, we discovered that the
latter two settings are kind of useless and are unlikely to satisfy anyone.
Instead, what some people will actually want is a way to return to the pre-Plasma
5.20 style, with one or two rows/columns of small-ish, fixed size icons.

This commit changes those two options to provide exactly that, allowing people who
don't like auto-scaling icons to return to the way their System Tray looked before
Plasma 5.20, and can also be used for global themes which are trying to emulate
Windows or Cinnamon, which have tray icons like this.