Integrate nclvalidator (C version) to ginga

[robertogerson]

Today, the most recent source code of nclvalidator (C version) is available at: https://github.com/TeleMidia/nclcomposer/tree/master/src/plugins/validator

One possibility is create a repo for nclvalidator and integrate it here (and to NCL Composer) through git submodules.

[alanlivio]

I suggest to study if is possible avoid bring QtXml and QtCore dependencies(present in nclcomposer validator) to ginga.

[robertogerson]

Yes, that is possible (and it is easy to do so)! The validator has an internal model that does not require QtXml/QtCore. Indeed, its original version supported both Xerces and Qt. But, since, to the date, we have been using it only in NCL Composer, we are maintaining only this version.

[alanlivio]

I also suggest rename src/nclconv to just src/parser.

[rafael2k]

Also, leave an option to run ginga without running the validator before execution.