Now state is now defined as a pseudoclass in the CSS format, selectors like:
GtkWidget:active { }
GtkButton:insensitive { }
GtkCalendar:prelight { } (also :hover is accepted)
...
define the style for the given state.
GtkCssProvider is a GtkStyleProvider implementation which is able to read
CSS-like input to style widgets.
At the moment, only the basic parser features are available.