qtui.h/noui.h interface cleanup

- rename wxMessageBox, remove redundant arguments to noui/qtui calls
- also, add flag to force blocking, modal dialog box for disk space warning etc
- clarify function naming
- no more special MessageBox needed from AppInit2, as window object is created before calling AppInit2

src/main.cpp

@@ -1811,7 +1811,7 @@ bool CheckDiskSpace(uint64 nAdditionalBytes)
         string strMessage = _("Warning: Disk space is low  ");
         strMiscWarning = strMessage;
         printf("*** %s\n", strMessage.c_str());
-        ThreadSafeMessageBox(strMessage, "Bitcoin", wxOK | wxICON_EXCLAMATION);
+        ThreadSafeMessageBox(strMessage, "Bitcoin", wxOK | wxICON_EXCLAMATION | wxMODAL);
         QueueShutdown();
         return false;
     }