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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. 
Can't this whole for-loop be done more efficiently using `findobj`? 
```
children = findobj(handle, 'type', 'text'); %NOTE: maybe even with extra arguments '-depth', 1
for child = children(:)'
    [m2t, str{end+1}] = drawText(child);
end
```
I don't know whether there can be multiple text children, but if there can be only one, the `for` loop above is not even needed.