The database helper and the skeleton of the data source are completed.
DueDate is added to the task model; additionally CreatedDate and
ModifiedDate fields are added to the database columns but not to the
task model in order to help sorting tasks by dates.
An activity separate from the main activity is considered for adding
tasks.
A listview of tasks are implemented for the MainActivity, however letsdo
crashes upon creation and I will remove this bug later.