app: allow filter actions to have hardcoded default operation settings
by encoding them directly in the string attached to all filter actions. The code now supports both "gegl:some-operation" and "gegl:some-operation\n<serialized config>". Add "default_settings" to GimpGeglProcedure to store the settings of the invoking action, much like the "default_run_mode" member. Change filters-commands.c to parse the new operation string, create GimpGeglProcedures with the deserialized settings, and use those settings when the procedures are ran. Change the filter history to be smarter about what is already in the history, there can now be several different procedures with the same name. Remove the dilate and erode actions from the drawable group, and add them to filters, they are just special cases of value-propagate with fixed settings.
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