Eclipse Xtext 2.26.0

This in mainly a maintence version to enhance compatibility with newer platform, jdt and java versions

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https://bugs.eclipse.org/bugs/buglist.cgi?bug_status=RESOLVED&bug_status=VERIFIED&bug_status=CLOSED&classification=Modeling&classification=Tools&columnlist=product%2Ccomponent%2Cassigned_to%2Cbug_status%2Cresolution%2Cshort_desc%2Cchangeddate%2Ckeywords&f0=OP&f1=OP&f3=CP&f4=CP&known_name=Xtext%202.26&list_id=16618269&product=TMF&product=Xtend&query_based_on=Xtext%202.26&query_format=advanced&status_whiteboard=v2.26&status_whiteboard_type=allwordssubstr