replace some ugly code in addressbookpage.cpp

- add signals signMessage() and verifyMessage() in addressbookpage.cpp
- connect to them in bitcoingui.cpp to switch to the corresponding tab in the Sign/Verify Message dialog
- make gotoSignMessageTab() and gotoVerifyMessageTab() private slots

src/qt/addressbookpage.h

@@ -76,6 +76,10 @@ private slots:
 
     /** New entry/entries were added to address table */
     void selectNewAddress(const QModelIndex &parent, int begin, int end);
+
+signals:
+    void signMessage(QString addr);
+    void verifyMessage(QString addr);
 };
 
 #endif // ADDRESSBOOKDIALOG_H