Added ConcurrentModify; Separated ConcurrentAdd

Previous ConcurrentToggleDone is now covered in ConcurrentModify.

Reverted a change in Storage interface: toggleDone method is now
removed. It is now handled again by modifyTask(chirptask.storage.Task).

Refactored some method names in TasksHandler class: addNotes -> setNotes
addDueDate -> setDueDate

Added some methods to TasksController and TasksHandler for the new
ConcurrentModify class.

Removed throwing some exceptions in some methods that did not need to
throw such as IOException in toggleTaskDone.

Removed protected access keyword where applicable to set to the default
(package) access.