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/util.cpp

@@ -1248,7 +1248,7 @@ void AddTimeData(const CNetAddr& ip, int64 nTime)
                     string strMessage = _("Warning: Please check that your computer's date and time are correct! If your clock is wrong Bitcoin will not work properly.");
                     strMiscWarning = strMessage;
                     printf("*** %s\n", strMessage.c_str());
-                    uiInterface.ThreadSafeMessageBox(strMessage+" ", string("Bitcoin"), CClientUIInterface::OK | CClientUIInterface::ICON_EXCLAMATION);
+                    uiInterface.ThreadSafeMessageBox(strMessage, "", CClientUIInterface::MSG_WARNING);
                 }
             }
         }