Replace QListWidget with vertical Layout to allow execution frame to be inserted

...rn_environment/include/learn_environment/subtask_item.hpp

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

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

catkin_ws/src/learn_environment/src/custom_list_widget.cpp

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