Revolved #37, #46, #54 + Fixed Undo/Redo +  Test Cases

#37: Implemented search function
#46: toDoTasks, doneTasks are now moved to DataFile.java
#54: Already fixed in previous commits.

> Undo/Redo for Delete search or Restore search now works consecutively.

Updated ProcessorTest to include more test cases as well as added a few
boundary test cases.