Further reduce header dependencies

This commit removes the dependency of serialize.h on PROTOCOL_VERSION,
and makes this parameter required instead of implicit. This is much saner,
as it makes the places where changing a version number can have an
influence obvious.

src/qt/walletmodel.cpp

@@ -4,6 +4,7 @@
 #include "addresstablemodel.h"
 #include "transactiontablemodel.h"
 
+#include "ui_interface.h"
 #include "wallet.h"
 #include "walletdb.h" // for BackupWallet