FR: Add slew bindings

"Slew" controls are those that affect the player's camera view angle without affecting the player's forward vector of the craft. Currently "Free look mode," also known as "mouse look mode" or similar in other games, does this ability but takes the main flight controls away from flying the ship. Making a dedicated set of slew controls would keep the flight controls flying.

VIEW controls differ from SLEW controls, in that the VIEW controls are locked to looking in a specific direction, while SLEW controls pan the camera view to an arbitrary angle.

SLEW_LEFT, SLEW_RIGHT, SLEW_UP, and SLEW_DOWN are for the digital controls.

Slew X axis and Slew Y axis are the analog controls.