[Feature Request] Allow binding of multiple axes to Analog Controls

A feature I really appreciate at the moment is being able to multiply bind controls to both DirectInput and XInput at the same time. This is convenient because it allows me to send my config.ini to a friend, and all their controls will be set up to work with an Xbox or Playstation controller without any further configuration required. I do some stream events with people who don't frequently use BizHawk, so I like to make sure the setup required for my participants is as small as possible, so this feature helps a lot.

But currently, there doesn't seem to be any way to bind multiple axes to Analog stick controls. I have to choose between XInput or DirectInput, which means I can't prepare it for someone beforehand without knowing whether they use an XInput controller or DirectInput controller. If we could have some way to multiply bind Analog controls like we do for buttons, that would be a huge help for me.

No files or tests are named in the issue. Start at BizHawk's controller configuration entry point and compare the existing multiple-button binding behavior with analog stick bindings. Done means an analog control can accept both DirectInput and XInput axes in one configuration, while preserving the current setup behavior.