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.