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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. 
Well, there is no real reason. I think based on get(0, 'ScreenPixelsPerInch') I somehow see inch as the default unit in this context. It's also the internal basis for convertUnits (for the same reasons). But you are right, for the tikz output, an SI unit might be nicer.