Complete sample parse functionality

Minor fixes/features:

- Result tree should quickly indicate whether there are any errors or not

- Parse must be executed against g* (as opposed to gTmp*) if grammar unchanged and no-previous error

Parse should indicate nicely if grammar problem hinders the sample parse (Button not selectable unless everything ok?!)

(from redmine: issue id 326, created on 2018-01-22tgradl, closed on 2018-01-22)

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