Issue 631: implement CSS background property

Reported by Nikolaus Schaller, Jul 7, 2014

1. the background: shorthand property is not implemented
2. the background-image property does not load any image
3. no background is drawn

A simple approach could be to draw the background image in 
-[_WebHTMLDocumentView drawRect:] but this is limited to the full 
screen background property.

If we want to set different backgrounds for each <div> 
(including <body>) we must implement that in a NSLayoutManager 
subclass that overwrites drawBackgroundForGlyphRange:atPoint:

This method should recognise a NSBackgroundImage attribute (and 
others like repeat) which are set in the _layout process where 
translation of CSS to NSAttributedString takes place.

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Small WebKit