Color picker: Add option to pick from all visible layers

[humdingerb]

Currently only the active layer is used for picking a color.

I first thought there should be a radio option like "All layers" or "Active layer". But is that actually needed? Normally you want to pick the colour under the mouse pointer and do not care on which layer it is. Otherwise, if you enter a transparent area on your layer, it just picks up alpha = 0 (as it does now).

[dsizzle]

The reason for such an option is to handle alpha scenarios. Imagine if you have 50% red on a layer over 100% blue. Do you always want the resulting purple color, or do you sometimes want the 50% red color?

especially with layer blend modes, do you want to only pick the blended result, or sometimes the actual non blended color?

[humdingerb]

Very good points! I'll change the ticket title...