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This pull request adds support for text arrows. Common code for the three drawing functions is now in one method.
Furthermore, a conversion for different positioning units is included in the merged text drawing. This allows to deal with second test figure from #394.

I think the conversion might cause some problems when the figure size in tikz differs from the figure size in Matlab. So in the future, a conversion to normalized or data coordinates might be better. 
Well, I never saw more than one and at the moment, I also can not think of a reason why there should be more. Are there some specification for this to make sure? Otherwise, I would just get rid of the loop.