ENH: fix generation of qgis descriptors

descriptors are generated with a custom target called
'generate_descriptors'. It depends on all OTBAPP.txt
So a missing .txt during installation will try to generate txt file.
This make sure all available applications have a valid descriptor.
There is a internal custom target called clean_descr which depends on
otbQgisDescriptor executable. This target re-create an empty directory
set in OTB_INSTALL_DESCR_DIR under OTB_BINARY_DIR. This make sure every
execution of generate_descriptor target is a clean one.

Finally, custom target is executed by make install using cmake's
install(CODE. Users can either can "make generate_descriptors" manually
or is automatically called during install.