Performed initial refactor of playWord() method

[pathumd]

Things completed in the BoardModel class:

• Performed initial refactor of playWord method where a new method called notifyInvalidPlacement() was created.
• This method takes in a message and creates a JOptionPane message dialog according to the message to notify the player of an invalid placement, and reverts the board to its initial state.
• This method is now invoked in the playWord method to reduce code duplication.
• Some changes were also made to the checkWordBonus method as well, including change of parameter and local variable names.