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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. 
Yes, I also think that this is not the most important issue.

Just an idea for discussion: One easy way for conversion would be to change the Units property of the figure objects temporarily. 
The main problem that I see here is, that this would mean to change the figure instead of just reading it.