camera_trigger : completely refactor state handling

msg/camera_trigger.msg

msg/vehicle_command.msg

src/drivers/camera_trigger/camera_trigger.cpp

src/drivers/camera_trigger/camera_trigger_params.c

src/modules/commander/commander.cpp

src/modules/mavlink/mavlink_mission.cpp

src/modules/navigator/mission_block.cpp

src/modules/navigator/mission_feasibility_checker.cpp

src/modules/navigator/navigation.h