Camera: Added feedback message support

Now the autopilot can publish the state of the camera via the mavlink_msg_camera_feedback message.
GCS_MAVLink: add CAMERA_FEEDBACK to enum
Copter: send a camera_feedback message when there is a camera trigger
Camera: record number of pictures taken since boot

The field is useful for detecting in a GCS if a mavlink_msg_camera_feedback message was dropped.
Copter: fix camera feedback msg handling

If the camera was disabled, camera message handling would fall though to
status-text msg handling.
Camera: initialise image_index
Camera: add f to float constants