Improved docstring for g.expand_css_constants.

Leo build: 20140914055338
Omg: a trivial change to the @data parser allows the user to compose @data node from the node and its descendants.
Replaced monolithic @data qt-gui-plugin-style-sheet with a tree of nodes!
Moved old @data node to the attic.

Leo build: 20140914055623
The parser for @data now ensure each node ends with a newline.

Leo build: 20140914060109