Create Landing Gear setup page (AC3.3)

Copter-3.3 includes support for Landing Gear but a number of people seem to have problems following the wiki instructions. http://copter.ardupilot.com/wiki/common-optional-hardware/landing-gear/

It just needs 3 controls:

Drop-down to specify which servo plugs into Pixhawk;
Set RCn_FUNCTION to 29 “Landing Gear”, where n is the number for the connected servo (e.g. RC9_FUNCTION for AUX1, RC10_FUNCTION for AUX2 etc.)

PWM selector for these two params:
LGR_SERVO_DEPLOY — servo PWM value required to deploy the landing gear.
LGR_SERVO_RTRACT — servo PWM value required to retract the landing gear.

Maybe it should have a note at the bottom telling the user to next go to the Extended Tuning screen and set one of the auxiliary switches (ch7, ch8) to Landing Gear ("29")

Start with the Landing Gear wiki instructions linked in the issue and inspect the existing Mission Planner setup pages to find the appropriate entry point. Add controls for the servo plug, deployment and retraction PWM values, and the optional Extended Tuning switch guidance; done means the page exposes the requested parameters clearly and matches the documented workflow.

Written by the indexing model from the issue text.