Implement getWidth method for the Word class

This method adds up the width of the Letters in the word. The width of the
word is added up to find the width of the line, and the width of the line
is used to determine whether the last word in the line should be moved to the
next line or not.
Remove unnecessary methods that change color of the Word

The purpose of the Word class is to serve as a container of the Letter class.
Although it may be useful in the future to add methods such as
"setColorOfNextLetter", it is useless to have methods that change the color of
all the letters in the word.