Whether colors may have alpha (translucency).
When ::use-alpha is %FALSE, the GdkRGBA struct obtained
via the [propertyGtk.ColorChooser:rgba] property will be
forced to have alpha == 1.
Implementations are expected to show alpha by rendering the color over a non-uniform background (like a checkerboard pattern).
The currently selected color, as a
GdkRGBAstruct.The property can be set to change the current selection programmatically.