plaintext.gen: use getPresentation() to return textual value of concepts like Line/VerticalLines

[arimer]

It is quite cumbersome to retrieve the actual string values from nodes of concepts like Line/VerticalLines, epecially when you want to setup proper "actual/expected tests". When using the getPresentation() for this purpose would be a nice possibility to retrieve those informations withouth having the need to run textgen.