Flightdata, Payload Control tab fixes

[rmackay9]

This fixes three issues I've found with the Payload Control tab

• The tab appears/disappears based on whether a MAVLink enabled gimbal has been seen or not but most gimbals are not mavlink enabled meaning this tab is normally invisible at first. I've removed the logic to show/hide this tab and instead it is left to the user to use the Customise checkboxes.
• The Roll scroll bar direction was reversed meaning a gimbal would roll in the opposite direction to what the user would expect
• MOUNT_CONTROL messages have been replaced by MAV_CMD_MOUNT_CONTROL commands. This command has been accepted since 4.0 (perhaps much earlier).

This has been tested on my PC with a DJI RS2 gimbal (see AP user wiki here) running latest ArduPilot. I have not tested with older versions of AP but I think it should all work.

The only thing I was not able to test was the case where the GCS sends lat,lon,alt commands to the gimbal. I think what I've done is correct though and from what I can see though this feature is never used in any case.

[rmackay9]

Thanks very much!