Respect word-wrapping when fitting text morph extent

The ``fit`` method of TextMorph always sets the whitespace handling to 'pre' which means that texts with normal word-break will not get the correct extent.

The following pictures shows a text morph with a background color. The extent of the text morph if off because the line wrapping was ignored:

![line-wrapping-bug](https://f.cloud.github.com/assets/479238/498644/2e81262e-bc1a-11e2-943e-2b4ddaef199f.png)