[pyqgis-console] Add targets and python script for generating console's QScintilla .pap file, for auto-completion

- Run 'make;make qsci-pap-src; make install' to update the default .pap file in source tree
- Run 'make;make qsci-pap-master[; make install]' for local-only .pap that overrides the default .pap (when testing new bindings)
- Include new qgis.core.NULL attribute defined in <src>/python/__init__.py for QPyNullVariant comparisons
- Update 11-month old default .pap