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/walletmodel.h

@@ -24,6 +24,7 @@ class WalletModel : public QObject
     Q_OBJECT
 public:
     explicit WalletModel(CWallet *wallet, OptionsModel *optionsModel, QObject *parent = 0);
+    ~WalletModel();
 
     enum StatusCode // Returned by sendCoins
     {
@@ -118,6 +119,8 @@ private:
     qint64 cachedNumTransactions;
     EncryptionStatus cachedEncryptionStatus;
 
+    void subscribeToCoreSignals();
+    void unsubscribeFromCoreSignals();
 signals:
     // Signal that balance in wallet changed
     void balanceChanged(qint64 balance, qint64 unconfirmedBalance);