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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 agree that inches are the standard for MATLAB, but this is only due to their American heritage (which we need not follow). In conversions, if you're working with an intermediate unit to convert to, that unit doesn't matter that much (i.e. you could use ancient Chinese units and nobody would notice). In our case `inch` the simplest option (since we handle `px`s and hence `dpi`s) for the intermediate step. But for the user-facing part, I'd personally use a saner standard because that's the de-facto standard in the world (and it makes sense from a scientific point-of-view, whereas inches, feet and yard are somewhat outdated IMHO).