update document thummbnail when a new page is inserted

src/board/UBBoardController.cpp

@@ -524,6 +524,7 @@ void UBBoardController::addScene()
     if (UBApplication::documentController->selectedDocument() == selectedDocument())
     {
         UBApplication::documentController->insertThumbPage(mActiveSceneIndex+1);
+        emit UBApplication::documentController->documentThumbnailsUpdated(UBApplication::documentController);
     }
 
     selectedDocument()->setMetaData(UBSettings::documentUpdatedAt, UBStringUtils::toUtcIsoDateTime(QDateTime::currentDateTime()));