Add topic indicator to UI and other small UI improvements

...c/learn_environment/include/learn_environment/task_ui.hpp

...in_ws/src/learn_environment/resource/learn_environment.ui

catkin_ws/src/learn_environment/src/learn_environment.cpp

catkin_ws/src/learn_environment/src/sidebar.cpp

catkin_ws/src/learn_environment/src/subtask_item.cpp

catkin_ws/src/learn_environment/src/task_manager.cpp

catkin_ws/src/learn_environment/src/task_ui.cpp

..._ws/src/learn_environment/task_pool/task_definitions.json