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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. 
I didn't find anything broken in the ACID, so that's good.

And since the MATLAB-approved way to deal with units is to set them back to the way you found them (i.e. you change them only temporarily such that you can convert some values or you can set the values in your preferred units), I think it's quite unlikely that many people will have problems with that. And at the moment, I think we've only once gotten a issue related to units.