qt: move export button to tabs

Having the export button at the top was confusing people into thinking
the entire wallet was exported.

This commit moves the export button to the address book, receiving
addresses and transaction tabs separately.

src/qt/bitcoingui.h

@@ -67,7 +67,6 @@ public:
     QAction * getAddressBookAction() { return addressBookAction; }
     QAction * getReceiveCoinsAction() { return receiveCoinsAction; }
     QAction * getSendCoinsAction() { return sendCoinsAction; }
-    QAction * getExportAction() { return exportAction; }
 
 protected:
     void changeEvent(QEvent *e);
@@ -98,7 +97,6 @@ private:
     QAction *receiveCoinsAction;
     QAction *optionsAction;
     QAction *toggleHideAction;
-    QAction *exportAction;
     QAction *encryptWalletAction;
     QAction *backupWalletAction;
     QAction *changePassphraseAction;