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/makefile.mingw

@@ -61,8 +61,7 @@ OBJS= \
     obj/sync.o \
     obj/util.o \
     obj/wallet.o \
-    obj/walletdb.o \
-    obj/noui.o
+    obj/walletdb.o
 
 
 all: bitcoind.exe