Copter: flight mode page always uses channel 5 regardless of FLTMODE_CH param

We should modify the FlightMode selector page to include a drop-down to allow users to set the FLTMODE_CH parameter and also update the green highlighting to reflect the channel that the user has selected.

I'm pretty sure this is working ok for Rover (which uses MODE_CH) and Plane so hopefully we can make the pages consistent.

Start at the Mission Planner FlightMode selector page and trace how the current channel 5 selection and green highlighting are produced. Compare the existing Rover MODE_CH and Plane pages, then verify that the page exposes FLTMODE_CH and highlights the user-selected channel consistently.

Written by the indexing model from the issue text.