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/wallet.cpp

@@ -276,7 +276,7 @@ bool CWallet::EncryptWallet(const SecureString& strWalletPassphrase)
         CDB::Rewrite(strWalletFile);
 
     }
-    NotifyKeyStoreStatusChanged(this);
+    NotifyStatusChanged(this);
 
     return true;
 }
@@ -1229,7 +1229,7 @@ string CWallet::SendMoney(CScript scriptPubKey, int64 nValue, CWalletTx& wtxNew,
         return strError;
     }
 
-    if (fAskFee && !ThreadSafeAskFee(nFeeRequired, _("Sending...")))
+    if (fAskFee && !uiInterface.ThreadSafeAskFee(nFeeRequired, _("Sending...")))
         return "ABORTED";
 
     if (!CommitTransaction(wtxNew, reservekey))