allow customizing the sorting of windows to allow row-major numbering

[dandrake]

See https://emacs.stackexchange.com/questions/76379/how-to-get-ace-window-to-do-row-major-numbering-of-windows/76380#76380 -- I'd like row-major window numbering when there's at least two rows and columns' worth of windows.

Consider a customization option to configure this behavior -- either a flag for row-major versus column-major ordering, or let users really go crazy and specify their own function to be used instead of aw-window<.