Convert UI interface to boost::signals2.

- Signals now go directly from the core to WalletModel/ClientModel.
  - WalletModel subscribes to signals on CWallet: Prepares for multi-wallet support, by no longer assuming an implicit global wallet.
- Gets rid of noui.cpp, the few lines that were left are merged into init.cpp
- Rename wxXXX message flags to MF_XXX, to make them UI indifferent.
- ThreadSafeMessageBox no longer returns the value `4` which was never used, converted to void.

src/qt/clientmodel.h

@@ -19,6 +19,7 @@ class ClientModel : public QObject
     Q_OBJECT
 public:
     explicit ClientModel(OptionsModel *optionsModel, QObject *parent = 0);
+    ~ClientModel();
 
     OptionsModel *getOptionsModel();
 
@@ -52,6 +53,8 @@ private:
 
     QTimer *pollTimer;
 
+    void subscribeToCoreSignals();
+    void unsubscribeFromCoreSignals();
 signals:
     void numConnectionsChanged(int count);
     void numBlocksChanged(int count, int countOfPeers);