share/otb directory contains i18n and maybe other data files for OTB.

This allows to simply some code.
* avoid search and install of i18n files in package scripts
* use of OTB_INSTALL_DESCR cmake variable for storing qgis description
* install qgis description to share/otb/description in all cases:
 ** build directory, otb, superbuild, remote modules?
 ** generate of missing description file when loading qgis plugin. (This
 always goes to share/otb/description)
 ** packaging in debian, ubuntu and binary packages
* unversioned otb doc folder. share/doc/OTB-X.Y -> share/doc/otb. This is where all VERSION , LICENSE are kept.

qgis descriptors are copied to share/otb/description.
This ends up with share/otb/ and share/OTB-X.Y
It is better to have a single directory for and it should be
share/otb and not a versioned. Changes included in packaging
script show non-versioned 'otb' directory in share is easy to
manage than a verisoned one. In any case, users cannot keep
multiple versions of OTB under same prefix.
This was breifly discussed in issue :
#1557

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.

otbsiftfast was enabling /fp:fast option with msvc along with SSE.
This is not normal as MSDN docs doesn't mention anything about /fp:fast
when using /arch:SSE.

I think it has a problematic behaviour with otb's dashboard
testing. Many --compare-image tests we do are with zero tolerance.
So it has to go.