Remove functions from they Python code for ViewProviderSpreadsheet that
are now in SheetView, and eliminate the unneded files for SheetView now
that it's being generated by hand to facilitate pseudo-inheritance.
* do not simply return with Py_None from Python handler without incrementing the ref. counter as otherwise the interpreter steals a reference and the program will crash after a while
* avoid using the plain Python C API and instead use the PyCXX API
Implement modifying the current selection programmatically via the
ViewProvider object in both C++ and Python. This enables unit testing of
GUI tasks that require a selection, and improves scriptability of
Spreadsheet.
