ENH: Restored the OFTString ase in GetFieldAsInt (this code snippet got accidentally erased in last commits))

ENH: Handle the new Integer64 field type introduced in gdal 2.0. Add default case at the end of switches to avoid warnings

BUG: MANTIS-1085; Added ::GetInput() method to otb::QtWidgetOutputFilenameParameter, otb::QtWidgetOutputImageParameter and otb::QtWidgetOutputVectorDataParameter.

ENH: Remove the TestCapability() method from the proxy namespace, has it is only needed for the Delete() method and does not exist anymore in gdal 2.x. Improved Delete() method instead, and simplified OGRDataSourceWrapper and OGRVectorDataIO code accordingly.