Add support for TreeGrid

[TatuLund]

I found that supporting TreeGrid should be rather straightforward, see discussion in the forum.

https://vaadin.com/forum/#!/thread/16971210

protected Integer addGridToExcelSheet(final Grid grid, final XSSFWorkbook myWorkBook, Sheet sheet,
int rowNum, final ExportExcelSheetConfiguration sheetConfiguration,
final ExportExcelComponentConfiguration componentConfiguration) needs to be changed to to fetch items via DataCommunicator instead of DataProvider for more universal compatibility with data sources.