Got customer side panel updating in real time.
Deactivate add button when quantity < 1. ActionListener instead of
MouseListener.
Checkout side panel updates in real time.
Increment and decrement buttons in checkout disabled/enabled as
required.
Seller side panel repainting enabled.
Merge branch 'newman' of https://github.com/jr-news/cop5339

Conflicts:
	src/shoppingCart/CustomerScreen.java