app: add the concept of line art source to Bucket Fill tool.
Additionally to sample merge and active layer, now we can only use the layer above or below the active layer as line art source. The line art fill is meant to work on drawing lines. Though sample merge still is ok in many cases, the more you fill with colors, the more the line art computation becomes unecessarily complex. Also when you use a lot of layers with some of them already filled with colors, it makes it impossible to colorize some line art zones with the tool. Moreover you just don't want to have to hide every layers out there to colorize one layer (especially background layers and such as you may want to see the result with your background). Thus we want to be able to set the source as a unique layer, while it not being necessarily the active one (because you want lines and colors on different layers). In this case, I am assuming that the color and the line layers are next to each other (most common organization). (cherry picked from commit c71b4916)
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