navigator: NavigatorMode and MissionBlock API cleanup

src/modules/navigator/loiter.cpp

src/modules/navigator/loiter.h

src/modules/navigator/mission.cpp

src/modules/navigator/mission.h

src/modules/navigator/mission_block.cpp

src/modules/navigator/mission_block.h

src/modules/navigator/navigator_main.cpp

src/modules/navigator/navigator_mode.cpp

src/modules/navigator/navigator_mode.h

src/modules/navigator/rtl.cpp

src/modules/navigator/rtl.h