Allow use of vJoy outputs in the axis merge tool

[WhiteMagic]

Currently only physical input axes can be merged via the merge axis option. In certain situations being able to merge vJoy axes of output devices could be useful. As the input selection is already manual via dropdown widgets adding the ability to use vjoy inputs should be doable.

[edwardwbarber]

In case this is useful for others, I wrote a short plugin to do something along these lines: here. It requires that both parent physical axes (in addition to the vJoy axes to combine) are specified by the user, since those generate callback events that trigger the axis update. This works well where intermediate vJoy axis is only being fed by one physical axis, since the intermediate vJoy axes aren't really meaningful without the input physical axes.

This approach does not work if a user wants to drive one intermediate vJoy axis (which is then fed into another vJoy axis) from two physical axes. To do that, seems like you would need vJoy devices to generate their own callback events.