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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. 
To me, exactly one makes a lot of sense. I don't think there is any documentation about the internal structure, but I think it's extremely unlikely that more text fields would be added. We can try to artificially put two or no text fields in there and see how the code reacts. I don't expect many problems, though.