Bitcoin-Qt: add "send coins" to context menu in addressbook

- allows to directly select an address from the addressbook, chose "send
  coins" from the context menu, which sends you to sendcoins tab and fills
  in the selected address

src/qt/sendcoinsdialog.h

@@ -29,6 +29,7 @@ public:
      */
     QWidget *setupTabChain(QWidget *prev);
 
+    void setAddress(const QString &address);
     void pasteEntry(const SendCoinsRecipient &rv);
     bool handleURI(const QString &uri);