Add CSS box model rendering

[Steditor]

Contents in websites should have correct borders, margins and paddings as well as a correct size.

Contains support and tests for:

• [x] display block
• [x] display inline
• [ ] padding
• [ ] margin
• [ ] border
• [x] width
• [x] height
• [x] position static

Estimated effort: 377

[rs22]

Width calculation still has some bugs (especially within tables). Widths of "replaced (inline) elements" (images, buttons) are ignored when calculating the "preferredMinimumWidth" of HTML elements (see also: http://www.w3.org/TR/CSS2/visudet.html). The image crashes when visiting some Websites (e.g. http://wikipedia.de), is this caused by the second pass of element width determination (updateDocumentSize -> setWidth)? Maybe there's an infinite loop? Could also be a SSL issue...