Improve generation of QGis descriptors

The current way to generate QGis descriptors is to call make generate_qgis_descriptor, after all applications have been generated. It would be more convenient to declare the descriptor file during otb_create_application macro. This way, each descriptor will be generated right after building the application, requiring a single call to make.

Of course, a boolean option in CMake can be added to enable/disable this automatic generation of descriptors.

Another proposal is to move the descriptor folder to PREFIX/lib/otb/descriptors.