move wallet code to separate file

This introduces two new source files, keystore.cpp and wallet.cpp with
corresponding headers. Code is moved from main and db, in a preparation
for a follow-up commit which introduces the classes CWallet and CKeyStore.

src/ui.h

@@ -4,6 +4,9 @@
 #ifndef BITCOIN_UI_H
 #define BITCOIN_UI_H
 
+#include <boost/function.hpp>
+#include "wallet.h"
+
 DECLARE_EVENT_TYPE(wxEVT_UITHREADCALL, -1)