Multiple bugs in camera gimbal config.

[AndKe]

Reproduce:
set some values for "stabilize" boxes, and servo min/max boxes.
observe that values are written and visible in full parameter list
exit application
start aplication, connect
observe that camera config PWM values are now defaults.

This means that visiting gimbal setup can destroy a working gimbal config by writing default values when one value is changed.

[billbonney]

This 7cb7f98 and 55cf467 with a592519 fixes most of the issues. I still have an issue where when you select say RC6 and it's already assigned, the UI doesn't show the old selection as disabled, but if you restart or refresh the params it shows as disabled.