`GtkSymbolicPaintable` is an interface that support symbolic colors in paintables.
`GdkPaintable`s implementing the interface will have the [[email protected]_symbolic] function called and have the
colors for drawing symbolic icons passed. At least 4 colors are guaranteed to be passed every time.
These 4 colors are the foreground color, and the colors to use for errors, warnings and success information in that order.