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/sendcoinsentry.h

@@ -26,6 +26,7 @@ public:
     bool isClear();
 
     void setValue(const SendCoinsRecipient &value);
+    void setAddress(const QString &address);
 
     /** Set up the tab chain manually, as Qt messes up the tab chain by default in some cases (issue https://bugreports.qt-project.org/browse/QTBUG-10907).
      */