app: allow picking layer in paint tools on alt-click.
When working with a lot of layers, it is common to have to switch easily between layers. And having to go back to the layer list is annoying and also sometimes not practical at all when you can't find easily the right layer. This is a first step in an experiment for such a feature, worked together with Aryeom as advisor (and originator of the feature idea). For now I apply this only to paint tools, though we are considering having it as a generic modifier too, working whatever the tool. Yet we wouldn't be able to use alt-left click (as it is used already in some tools). How it works is simply that in any paint tool, alt-click allows to switch to the topmost layer having a visible pixel at the clicked position.
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