Honor alpha channel in most colors

[Geokureli]

Flixel uses ARGB colors everywhere but usually has a color and alpha field anyway, this is redundant. We should honor the alpha channel of a color, especially in existing cases where there is no corresponding alpha field for some color property. One example is FlxInputText.selectionColor.

In cases above, where we enable the alpha component, we should enable it for all colors in that class, for consistency. Even if the others have a separate alpha field, we should multiply both for backwards compatibility and deprecate the alpha field, pointing towards the FlxColor's alphaFloat.