Add new string members to GimpImageMapTool and use them instead of the
resp. fields of GimpToolInfo. Change ::get_operation() to return the
operation name and a lot of strings for the UI, and create both the
GeglNode and the config object in GimpOperationTool. Lots of various
cleanups in GimpImageMapTool subclasses. This is an intermediate state
on the way of making the whole filter applying mechanism more generic
and less depending on subclasses.