Added a lot more functionality to BookingScreen and to SeatChartScreen
as well as various other classes in order to make almost all features of
selecting a specific seat work.

Also, implemented a method that finds the first open/available seat if
the customer chooses not to select a specific seat and instead just let
the program choose an available seat for him/her.

Fixed some equality checks in Manifest that were using == instead of
.equals().

Modified Receipt slightly to print out nicer and also include the seat
number the customer has booked. Additionally updated ReceiptTest to
reflect the Receipt changes.

BookingScreen customer input info no longer gets lost when selecting a
seat via the display seating button and then returning to BookingScreen.
Also, BookingScreen tells the user which seat they will currently be
booking if he/she doesn't change it in SeatChartScreen before clicking
the book flight button.

Added some Error windows for when user enters invalid info into various
user input text boxes on various screens. Still more may need to be
added in the near future.

Did some other minor edits to various files to make everything work
right!!!

One final note: Although a lot of functionality works properly now, the
booking itself does not seem to be saved and remembered by the program
because after booking a seat on a flight, that seat is back to being
available the next booking I try to do, and shows up as empty on the
seating chart even though I modified the code to show which seats are
already occupied in the seating chart. It's as if the database gets
cleared every time you complete one booking and the program takes you
back to MenuScreen. This will need to be fixed at some point. Maybe
during tomorrow's meeting!!!