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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. 
Hmm, now I come to think of it: this looks a bit odd together with the `otherwise` case or `normalized` case above: the `X/YLim`s are in `data` units, so the `pos` should as well (but this isn't always the case). We should fix this (even though I merged this PR already)