Mouse support resize-with-center

So in default configuration there is this option:

`yabai -m config mouse_action2 resize`

Which works well however as you are resizing you are only controlling x-y axis increase in one direciton at once, it would be awesome if there was an option like when you take a corner of a window and resize it while holding alt key. (I'm assuming this is native behaviour, if not basically when you resize a corner while holding alt you are stretching/contracting x-axis and stretching-contracting y-axis). This functionality is only really useful for floating windows but I feel like it would be a logical addition, not sure exactly how the cursor should be handled....)

Start by locating the mouse_action2 resize handling in the C source and trace how floating-window cursor movement is processed. Compare the current one-axis behavior with the requested modifier-based corner resize, including the unresolved cursor behavior. Done means a documented option or interaction resizes both axes together for floating windows without breaking existing resize behavior.

Written by the indexing model from the issue text.