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/bitcoinrpc.cpp

@@ -769,7 +769,7 @@ void ThreadRPCServer2(void* parg)
                 strWhatAmI.c_str(),
                 GetConfigFile().string().c_str(),
                 EncodeBase58(&rand_pwd[0],&rand_pwd[0]+32).c_str()),
-            _("Error"), CClientUIInterface::OK | CClientUIInterface::MODAL);
+                "", CClientUIInterface::MSG_ERROR);
         StartShutdown();
         return;
     }
@@ -860,7 +860,7 @@ void ThreadRPCServer2(void* parg)
     }
 
     if (!fListening) {
-        uiInterface.ThreadSafeMessageBox(strerr, _("Error"), CClientUIInterface::OK | CClientUIInterface::MODAL);
+        uiInterface.ThreadSafeMessageBox(strerr, "", CClientUIInterface::MSG_ERROR);
         StartShutdown();
         return;
     }