MavCmd: alphebetise and fixup rover command list

[rmackay9]

This PR makes a few changes primarily to the Rover command list (there is one change to the Plane list)

• alphebetise the rover command list. Like Copter's list the nav commands appear at the top with the two most common commands (WAYPOINT and RETURN_TO_LAUNCH) appearing right at the top. Other less popular nav commands are then in alphabetical order. "do" and "conditional" commands are next and are all in alphabetical order.
• removed unsupported do-set-home
• removed unsupported do-mount-configure from both rover and plane's lists.
• added titles to the do-digicam-configure and do-digicam-control messages (for rover)
• removed unsupported arguments from do-change-speed (for rover)

I have tested this on my local machine by copying the mavcmd.xml and confirming the commands appear as expected

[rmackay9]

Thanks!