qtui.h/noui.h interface cleanup

- rename wxMessageBox, remove redundant arguments to noui/qtui calls
- also, add flag to force blocking, modal dialog box for disk space warning etc
- clarify function naming
- no more special MessageBox needed from AppInit2, as window object is created before calling AppInit2

src/qt/walletmodel.cpp

@@ -140,7 +140,7 @@ WalletModel::SendCoinsReturn WalletModel::sendCoins(const QList<SendCoinsRecipie
             }
             return TransactionCreationFailed;
         }
-        if(!ThreadSafeAskFee(nFeeRequired, tr("Sending...").toStdString(), NULL))
+        if(!ThreadSafeAskFee(nFeeRequired, tr("Sending...").toStdString()))
         {
             return Aborted;
         }