Related to #3159 and #3160 (same changes for transactionview and walletview).
Goal: Harmonize user experience for export of addresses, transactions and backup of the wallet!

- allow to use message() in addressbookpage by connecting to the
message() signal in WalletView
- needs to change the model in setModel() from AddressTableModel to
WalletModel
- use message() for displaying success or failure of export
- rework the strings to be more detailed / informative
- additional small cleanups
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As I mentioned somewhere else: AddressBookPage is supposed to be a standalone dialog that can be launched from anywhere, so I'd prefer not making it dependent on connecting signals.
Yeah, I'm going to rework this, to not use mesage(), that way as it seems impossible to do it in a clean way and it's a standalone dialog, as you also said. It's a tad sad that my invention breaks the coding-rules somehow, but I'm happy you appreciated this :-).
What about a message() function (not signal) in GUIUtil, perhaps nearly a copy of the one in BitcoinGUI (so we have a unified user experience and API)? Just a quick thought...
Closing this for now