camera feedback mavlink messages

CAMERA_EVENT and CAMERA_FEEDBACK messages, and a couple of enums they
use (CAMERA_EVENT_TYPES and CAMERA_FEEDBACK_FLAGS)

Adjusted some types and added more explicit descriptions of units as
suggested on the call last week.

I will add camera parameter list/get/set functionality as suggested
using the existing parameter mechanism and component IDs rather than
the new CAMERA_INFO messages I had proposed.