Enhancement: Support Grammars Extending Other Grammars

[david-pace]

When generating a web editor for a grammar depending on another grammar like this:

grammar tld.domain.my.dsl.MyExtendingDSL with tld.domain.my.dsl.MyBaseDSL

errors like the following are thrown during generation:

java.lang.IllegalStateException: Problem parsing 'platform:/resource/tld.domain.my.dsl/src/main/java/path/to/package/dsl/MyExtendingDSL.xtext': XtextLinkingDiagnostic: null:1 Couldn't resolve reference to Grammar 'tld.domain.my.dsl.MyBaseDSL'.