Make accessory 3 to 5 usable

#1010

This is a new try to create the Pull Request, without my errors from the previous try.

Reason for this PR:
In Mixer we can use Asseccory 0 to 5, but in GCS you can only setup 0 to 2
Even thru UAVObject Browser we only can setup Accessory 0 to 2
I like to setup the missing channels in the GCS.
Also I want to use Asseccory 3 to 5 for mixer.
In the moment we only can use 3 additional Rx channels.

With this PR we can use up to 6 additional Rx channels.
For all my wanted features I need more than the actual the actual 3 channels ;)

3 for Gimbal
1 for controlling camera (take photo)
2 for MWC-Light RGB Controller (http://www.hadi-rc.de/MWC-Light-RGB-Contro)