Add support for text shadows

Add support for text shadows, ideally based on the CSS level 3 'text-shadow' property.

This website has examples of what can be accomplished:
http://www.w3.org/Style/Examples/007/text-shadow

Shadows would be a character property and specified by a list of a set of character properties (offset-x, offset-y, blur radius, and color). Multiple shadows may be specified/drawn per character (as per the 'text-shadow' specification).

This change would involve extending QTextFormat and QTextCharFormat to record the shadow format for text characters. It would also involve several changes to the QTextDocument group of classes to record, store, and draw shadowed characters.

Character properties are currently treated as attributes (i.e. a given property has a single value). Since 'text-shadow' may not follow this pattern (it would have a list of shadows), it would be acceptable to have a fixed maximum number of shadows, perhaps four (4), to facilitate the drawing letters as outlines.