Added three new widgets:
    1) WalletView - container for overviewPage, transactionsPage, addressBookPage, receiveCoinsPage, and sendCoinsPage for a single wallet
    2) WalletStack - container for WalletView objects, one WalletView per wallet
    3) WalletFrame - central widget in BitcoinGUI, owns an instance of WalletStack and passes all messages from GUI to wallets.