app: cleanup in GimpImageMap

- don't allow to create a GimpImageMap of an operation without output
- make "region", "mode" and "gamma-hack" settable on a map that
  already has a graph
- don't insert a useless "over" if the operation is a source op
- do the gamma-hack always on formats with alpha, so we don't lose
  intermediate alpha results on source ops
- simplify graph connection a lot
- in GimpImageMap tool, don't recreate the map when reconfiguring
  "region" and "gamma-hack"