editor listener wasn't wiped from associated user data; rm'd 2x; make lexer save update preview and reload associated lg in parser's previewState. almost there...doesn't compile.
fan out from using VirtualFile not filename in PreviewState
must load parser if lexer changes.
disable preview window for lexers. Fixes #141
don't parse if bad lexer/grammar. only update parse tree, errors if showing non-lexer.
tweak
Merge pull request #142 from parrt/fix-sep-grammars

Fix separate lexer/parser grammars. Fixes #138, Fixes #141