Makeover and education

Makeover:
MainWindow sizers added (grid & box sizer);
all windows resized; unnecessary text removed; buttons repositioned;
FileWindow created; menus rationalised (fewer in TrainingWindow, more
in MainWindow); second button added to MainWindow for demonstration
purposes; ComboBox added to MainWindow as possible alternative to
buttons (not to be present together in final product); language “eng”
renamed to “British English”
Education:
“Add” feature added to file menu; TrainingWindow button behaviour
rationalised; method suggested (commented out, lines 116-120) for
avoiding multiple TrainingWindows (not yet functional); database
integration begun