Sequence Writer crashes when GFF fromat is chosen

Sequence Writer provides choosing GFF format, but it is not implemented now. So, UGENE crashes on assert(0).
Implement writing of GFF files.

Currently there is a hardcoded solution in the "CoreLib.cpp" file (at branch):

---------------------------------------------------------
// temporary excluding GFF format (UGENE-747)
{
supportedFormats.removeOne(BaseDocumentFormats::GFF);
}
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Remove it after implementing this issue.