Alpha channel for RGB node to support (semi)transparent swatches

[MathemanFlo]

ArmorPaint and Blender differ in their handling of alpha values. While Blender treats them as parts of colors ArmorPaint treats them separately. Imho the ArmorPaint way of doing it is great because it is not clear for all nodes how they would behave on alpha values. For instance a gamma correction, does it affect the alpha or not. @luboslenco What do you think about my idea to

1. Add a alpha output slot to the RGB node (similar to color ramp node)
2. Enable alpha selection for RGB node
3. Enable alpha selection for swatches

And therefore allow swatches to have alpha transparencies. In case you like this idea I would like to implement it. Furthermore it would be great to get some input about #1263 before I start because in this case I would also update the "Add swatch" to support opacity as well.

Optionally one could also add a button to the RGB node for creating a new swatch. This could be a bit probelmatic as the nodes are defined in Armorbase.