Allow user to individualize settings for each layer

[khalim19]

To allow greater flexibility, let the user to customize settings for each layer separately. For example, the user would want to enable a setting (e.g. "Autocrop") for all layers except one. While it could be done with some workarounds (e.g. export again with autocrop disabled), they are quite inconvenient. This information should be stored somewhere in the layers/image so as to preserve it between GIMP sessions.

[khalim19]

With the added support for custom layer tags and the upcoming support for custom operations, users may be able to apply specific operations to an arbitrary group of layers (defined by constraints, including custom tags). In other words, different operations can be possibly applied to each layer.

[kamilburda]

One way to implement this is to reuse the constraints and allow them to be applied to individual procedures. That way, a procedure will be applied to a subset of layers (further reducing the subset given by the "global" constraints).