Alpha slider for color
Description of the feature
A slider, present in the color dialog , that lets the user choose the Alpha value of the FB/BG colors (in addition to the RGB/HSV/etc values).
Currently, the only way to choose to Alpha value of the color being painted is by adjusting the opacity slider on the tool being used. The opacity slider on the Layers dialog is very useful for visualization, but it should not be used in the Tool Options dialog for two reasons:
Opacity is not a proper representation of Alpha - Alpha uses a 0-255 scale while opacity uses a 0-100 scale.
The transparency of a pixel is a property of the pixels themselves, not the tool used to paint it. Therefore it should not be controlled through the tool window, but the color window instead.
Many image editing programs allow this to be done, such as Paint.NET or Aseprite:
The opacity slider on the Layers dialog should also potentially use a 0-255 scale, although this should be a separate issue.
Additionally, the color picker should also pick the alpha value of the pixel being picked.