CardReader: made attribute 'saving' private and added a getter for it, added endSaving() method.

CardReader: added some debug code for file open/close count.

UltiLCD2_menu_print.cpp: revert 86b7c9f0bd4ed34c956cdce2d46ae7441fac9cd9, this caused a error at
printing end.

CardReader:printingHasFinished(): use closefile() instead of file.close() because of open-count.

Fixed typo and added some documentation.