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/util.cpp

@@ -1000,7 +1000,7 @@ void AddTimeData(const CNetAddr& ip, int64 nTime)
                     string strMessage = _("Warning: Please check that your computer's date and time are correct.  If your clock is wrong Bitcoin will not work properly.");
                     strMiscWarning = strMessage;
                     printf("*** %s\n", strMessage.c_str());
-                    ThreadSafeMessageBox(strMessage+" ", string("Bitcoin"), wxOK | wxICON_EXCLAMATION);
+                    uiInterface.ThreadSafeMessageBox(strMessage+" ", string("Bitcoin"), MF_OK | MF_ICON_EXCLAMATION);
                 }
             }
         }