Fix the issues with the computation of the editor height 

I finally find a way to first, accurately compute the heigh of the editor, and second update the scrollbar when the content change even if an image or a font take a few seconds to load.

To show the problem with the current implementation, set the content of the editor with images to overflow the editor and finish with a br. Example:

    <img src="a-big-img.jpg" width="2000" height="2000"><img src="a-big-img.jpg" width="2000" height="2000"><br>