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