Qt5 compatibility

This commit squashes all the changes in the Qt5 branch
relative to master.

Backward compatibility with Qt4 is retained.

Original authors:

- Philip Kaufmann <phil.kaufmann@t-online.de>
- Jonas Schnelli <jonas.schnelli@include7.ch>

src/qt/walletframe.h

@@ -17,8 +17,9 @@ class WalletStack;
 class WalletFrame : public QFrame
 {
     Q_OBJECT
+
 public:
-    explicit WalletFrame(BitcoinGUI *_gui);
+    explicit WalletFrame(BitcoinGUI *_gui = 0);
     ~WalletFrame();
 
     void setClientModel(ClientModel *clientModel);