implement subtask execution status and logging in the UI

...n_environment/include/learn_environment/execute_frame.hpp

...n_environment/include/learn_environment/script_worker.hpp

.../src/learn_environment/include/learn_environment/task.hpp

catkin_ws/src/learn_environment/resource/icons/loading.gif

catkin_ws/src/learn_environment/resource/icons/queued.gif

catkin_ws/src/learn_environment/resources.qrc

catkin_ws/src/learn_environment/src/notebook_converter.cpp

catkin_ws/src/learn_environment/src/script_worker.cpp

catkin_ws/src/learn_environment/src/subtask_item.cpp

catkin_ws/src/learn_environment/src/task_manager.cpp