Remove unused constructor in Letter class

Using charAt() for unchecked String con potentially cause
StringIndexOutOfBoundsException.
Pass Preferences to the Letter class when creating new lines/words/letters

Getting the Preferences object by using a static method is useless. The
parameters for the split() method is not changed because this method will be
removed in the next commit.
Delete split() method in Word class

The method not only splits the word but also adds the letters to JPanel.
Rather using a separate method to do a simple for-loop, the process is moved
to the constructor for better understandability.
Add class that shows a list of records
Add Timestamp to Record class so that it can be stored when getting records from database

Although Timestamp is not needed when adding to database because it gets added
automatically, it is needed when retrieving from database.