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.h

@@ -70,8 +70,8 @@ signals:
     void numBlocksChanged(int count, int countOfPeers);
     void alertsChanged(const QString &warnings);
 
-    //! Asynchronous error notification
-    void error(const QString &title, const QString &message, bool modal);
+    //! Asynchronous message notification
+    void message(const QString &title, const QString &message, bool modal, unsigned int style);
 
 public slots:
     void updateTimer();