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.