Annotations

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 tried to remove or add text fields, but Matlab rejected to do so. Thus, I assume it's fine to remove the loop.