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/main.cpp

@@ -2197,10 +2197,10 @@ bool CheckDiskSpace(uint64 nAdditionalBytes)
     if (nFreeBytesAvailable < nMinDiskSpace + nAdditionalBytes)
     {
         fShutdown = true;
-        string strMessage = _("Warning: Disk space is low!");
+        string strMessage = _("Error: Disk space is low!");
         strMiscWarning = strMessage;
         printf("*** %s\n", strMessage.c_str());
-        uiInterface.ThreadSafeMessageBox(strMessage, "Bitcoin", CClientUIInterface::OK | CClientUIInterface::ICON_EXCLAMATION | CClientUIInterface::MODAL);
+        uiInterface.ThreadSafeMessageBox(strMessage, "", CClientUIInterface::MSG_ERROR);
         StartShutdown();
         return false;
     }
@@ -3681,7 +3681,6 @@ public:
 
 CBlock* CreateNewBlock(CReserveKey& reservekey)
 {
-
     // Create new block
     auto_ptr<CBlock> pblock(new CBlock());
     if (!pblock.get())