Implement getWordsWidth method

This method returns the width of the line in terms of the total length of the
words and the spaces between the words. This information cannot be retrieved
by using the inherited getWidth method, which simply returns the width of the
line.
Change Word class to extend JPanel

Due to this, the getWidth method becomes unnecessary because:
1. It overrides the getWidth method of JPanel
2. The total width of the JPanel is always the sum of the Letters
Add Look and Feel of native operating system

The look and feel of the application is set to the native OS if there is one,
and set to the default (JLF) if not.
Rename ambiguous variable name "word" to "letters"

The variable is renamed because it is better to call an ArrayList of Letter
"letters" rather than "word".