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/keystore.h

@@ -8,6 +8,7 @@
 #include "crypter.h"
 #include "sync.h"
 #include "base58.h"
+#include <boost/signals2/signal.hpp>
 
 class CScript;
 
@@ -174,6 +175,11 @@ public:
             mi++;
         }
     }
+
+    /* Wallet status (encrypted, locked) changed.
+     * Note: Called without locks held.
+     */
+    boost::signals2::signal<void (CCryptoKeyStore* wallet)> NotifyStatusChanged;
 };
 
 #endif