Load file has new importing parameter for sy-batch

In the interface method ZIF_EXCEL_READER~LOAD_FILE, add an optional
import parameter I_FROM_APPLSERVER of type flag with default SY-BATCH,
and use this parameter, instead of SY-BATCH directly, for the decision
whether to load the file from application or from presentation server.
This will make the method more flexible, decoupling the decision "Where
to read the file from" from the circumstance "Am I in batch mode". By
making the parameter optional and using the default value SY-BATCH, all
existing client code will work as before.