update CClientUIInterface and remove orphan Wx stuff

- fix ThreadSafeMessageBox always displays error icon
- allow to specify MSG_ERROR / MSG_WARNING or MSG_INFORMATION without a
  custom caption / title
- allow to specify CClientUIInterface::ICON_ERROR / ICON_WARNING and
  ICON_INFORMATION (which is default) as message box icon
- remove CClientUIInterface::OK from ThreadSafeMessageBox-calls, as
  the OK button will be set as default, if none is specified
- prepend "Bitcoin - " to used captions
- rename BitcoinGUI::error() -> BitcoinGUI::message() and add function
  documentation
- change all style parameters and enum flags to unsigned
- update code to use that new API

- update Client- and WalletModel to use new BitcoinGUI::message() and
  rename the classes error() method into message()
- include the possibility to supply the wanted icon for messages from
  Client- and WalletModel via "style" parameter

src/qt/clientmodel.cpp

@@ -84,7 +84,7 @@ void ClientModel::updateAlert(const QString &hash, int status)
         CAlert alert = CAlert::getAlertByHash(hash_256);
         if(!alert.IsNull())
         {
-            emit error(tr("Network Alert"), QString::fromStdString(alert.strStatusBar), false);
+            emit message(tr("Network Alert"), QString::fromStdString(alert.strStatusBar), false, CClientUIInterface::ICON_ERROR);
         }
     }