Return Window, ContainerControl, and ColourPicker colour properties as positive packed ARGB values

Comparing identical colours for equality can return False even though the colours are the same. After assigning Colour.RGB(204, 51, 102) to a Window's BackgroundColour (or a ContainerControl's, or reading the same colour back from a ColourPicker's SelectedColour), comparing the property against that same colour returns False, and comparing read-backs across different controls returns False too. The affected properties store the colour in a signed form internally, so their read-back values differ from the value every other colour property and every Colour factory returns for the same colour.

Reading a colour property now returns the same value everywhere: the positive packed ARGB form, identical to what Colour.RGB, Colour.RGBA, and the other colour properties return. Identical colours compare equal no matter which property produced them, and the value read back from a property equals the value assigned to it.