Remove temporary fix for null in Task.dateList
Implement Task.compareTo() that supports all three types of tasks
Hotfix for missing import in Logic.java
Simplify Task.isDateListEmpty()
Rename Task.isDone() back to Task.getIsDone() as the renaming broke Java Bean XML converter
Let DatabaseManager provide a comparator to compare two databaseId
Refactor Logic to use comparator provided by dbManager and switch to displayedTasksList instead of a Map
Move validating of displayedId to a method