Quadplane: Allow rudder to operate in VTOL modes

This request was made in a [forum post](http://discuss.ardupilot.org/t/questions-and-observations-from-my-quadplane-maiden/10035/3) and I wanted to add it here for tracking.

Question: Why does the rudder not operate in Q modes? Wouldn't it help to yaw the aircraft using the wind?

@tridge: "you are right that we could use rudder in QHOVER and QLOITER, although I don't think it would be very effective at low speeds. It is probably worth a try though. The trick will be getting the gain right. I'll see if I can come up with a reasonable approach."

Start by tracing how rudder and yaw control are handled in the quadplane's QHOVER and QLOITER modes, using the linked forum discussion for the intended behavior. Determine how rudder input should be applied and how its gain should be selected; done means rudder can yaw the aircraft in the relevant VTOL modes without disrupting existing control.

Written by the indexing model from the issue text.