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/net.cpp

@@ -705,7 +705,7 @@ void ThreadSocketHandler2(void* parg)
         if (vNodes.size() != nPrevNodeCount)
         {
             nPrevNodeCount = vNodes.size();
-            NotifyNumConnectionsChanged(vNodes.size());
+            uiInterface.NotifyNumConnectionsChanged(vNodes.size());
         }