Don't overwrite notification flags while setting a title for wxProgressDialog (wxMSW)

In wxProgressDialog::SetTitle(), wxSPDD_TITLE_CHANGED flag signaling a request to update a title should be added to the flags already being set and signaling another data waiting for the update.

Closes #17966.

src/msw/progdlg.cpp

@@ -626,7 +626,7 @@ void wxProgressDialog::SetTitle(const wxString& title)
         {
             wxCriticalSectionLocker locker(m_sharedData->m_cs);
             m_sharedData->m_title = title;
-            m_sharedData->m_notifications = wxSPDD_TITLE_CHANGED;
+            m_sharedData->m_notifications |= wxSPDD_TITLE_CHANGED;
         }
     }
 #endif // wxHAS_MSW_TASKDIALOG