- added some nullptr checks to SFTPSession, SFTPFile and SFTPDir

- now returning pointers instead of objects when creating new SFTPSession, SFTPFile and SFTPDir objects
- removed some debug messages
- changed example program to use the new methods (dir example now works -- no more crashing)

- removed APTEnsWidget and APTEnsDataModel from the example (not relevant to it and previously moved to another project)
- more changes to support SFTPFile and SFTPDir usage
- added functionality to GUI to demonstrate SFTPFile and SFTPDir usage (although the latter is not yet working)

- need to finish implementation and separate into new source and header files

#1. Initial implementation of Session and Channel classes for separation of session and execution channels to support multi-threaded environment.