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 - takes the place of the central widget in BitcoinGUI, owns an instance of WalletStack and passes all messages from GUI to wallets.