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/bitcoingui.h

@@ -119,8 +119,14 @@ public slots:
     */
     void setEncryptionStatus(int status);
 
-    /** Notify the user of an error in the network or transaction handling code. */
-    void error(const QString &title, const QString &message, bool modal);
+    /** Notify the user of an event from the core network or transaction handling code.
+       @param[in] title     the message box / notification title
+       @param[in] message   the displayed text
+       @param[in] modal     true to use a message box, false to use a notification
+       @param[in] style     style definitions (icon and used buttons - buttons only for message boxes)
+                            @see CClientUIInterface::MessageBoxFlags
+    */
+    void message(const QString &title, const QString &message, bool modal, unsigned int style);
     /** Asks the user whether to pay the transaction fee or to cancel the transaction.
        It is currently not possible to pass a return value to another thread through
        BlockingQueuedConnection, so an indirected pointer is used.