Refactor getIsDone to isDone for task
Logic change, task with no date are not shown

task with no date are not shown during view by single date or view by
period.
Implemented Comparable to Task

Tasks produce from view by period are now sorted according to date
Remove temporary fix for null in Task.dateList
Implement Task.compareTo() that supports all three types of tasks
Merge branch 'quality_control'
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
Merge branch 'quality_control'