Simplify VIEW & SEARCH methods
Add a helper method for MARK command

* Move validity of displayedId to the new method (the old one actually does not work because NullPointerException is thrown as early as in isCompletedTask)
Change LogicTest (due to changes in VIEW that it accepts a boolean isCompleted)
Refactor Logic.updateTask() to make it neater
Rename Logic.delete to Logic.deleteTask
Merge branch 'quality_control'