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/noui.cpp

@@ -1,59 +0,0 @@
-// Copyright (c) 2010 Satoshi Nakamoto
-// Copyright (c) 2009-2012 The Bitcoin developers
-// Distributed under the MIT/X11 software license, see the accompanying
-// file license.txt or http://www.opensource.org/licenses/mit-license.php.
-#include "ui_interface.h"
-
-#include <string>
-#include "init.h"
-
-int ThreadSafeMessageBox(const std::string& message, const std::string& caption, int style)
-{
-    printf("%s: %s\n", caption.c_str(), message.c_str());
-    fprintf(stderr, "%s: %s\n", caption.c_str(), message.c_str());
-    return 4;
-}
-
-bool ThreadSafeAskFee(int64 nFeeRequired, const std::string& strCaption)
-{
-    return true;
-}
-
-void InitMessage(const std::string &message)
-{
-}
-
-std::string _(const char* psz)
-{
-    return psz;
-}
-
-void QueueShutdown()
-{
-    // Without UI, Shutdown can simply be started in a new thread
-    CreateThread(Shutdown, NULL);
-}
-
-void NotifyBlocksChanged()
-{
-}
-
-void NotifyKeyStoreStatusChanged(CBasicKeyStore *wallet)
-{
-}
-
-void NotifyAddressBookChanged(CWallet *wallet, const std::string &address, const std::string &label, EntryStatus status)
-{
-}
-
-void NotifyTransactionChanged(CWallet *wallet, const uint256 &hashTx, EntryStatus status)
-{
-}
-
-void NotifyNumConnectionsChanged(int newNumConnections)
-{
-}
-
-void NotifyAlertChanged(const uint256 &hash, EntryStatus status)
-{
-}