Remove content.normalizeSpaces()

The method `content.normalizeSpaces` aims to convert the first and last space to a non breaking space charcter to prevent visual collapse by some browsers. But this causes  whitespace to become visible if there is whitespace before the Text in the markup.

The appearance of a paragraph like the following with whitespace around the text will change visually after normalizeSpaces has been executed:

```html
<p>   Test   </p>
```

We need a better solution to prevent empty contenteditables to collapse (take zero height). In Livingdocs the placeholders fullfill this job but in the default editable setup this issue is not taken care of.