This type defines a gradient between 2 symbolic colors, with any number
of color stop between these. At the moment it has been put besides
GtkSymbolicColor, although should be completely private, it will likely
need extending in the future for radial gradients.
At some point, it maybe should also be used for images, so painting
both a gradient and an image is consistent.
This support goes from the theming engines, which are able to retrieve
style for different combined states to the CSS provider, where several
state pseudo-classes may be specified, such as:
GtkButton:active:prelight {}
This will set the default value to a given property for all possible states,
such default value will only be used if the queried state doesn't have a
value itself.
