Added saveState and loadState methods (fully working, but missing
settings saving). Edited context_menu_delete so it could have a second
menuItem (Details, shows the other course details such as course
schedule and teacher, etc.). Renamed to context_menu only.
Added NotificationService class that will handle the background service
for periodical notifications. //TODO: complete the service and edit
loadState and saveState methods so the settings can be saved.