GIMP's OVERLAY mode was identical to SOFTLIGHT. This commit fixes the
issue and introduces a NEW_OVERLAY mode and enum value.

- change gimp:overlay-mode to be a real (svg-ish) overlay mode
- when compositing, map OVERLAY to gimp:softlight-mode
- when compisiting, map NEW_OVERLAY to gimp:overlay-mode
- bump the XCF version when NEW_OVERLAY is used
- map OVERLAY to SOFTLIGHT when loading and saving XCF
- map OVERLAY to softlight in all PDB setters
- map OVERLAY to softlight when deserializing a GimpContext
- change all paint mode menus to show an entry for NEW_OVERLAY
  instead of OVERLAY
- change PSP, PSD and OpenRaster to use NEW_OVERLAY

These changes should (redundantly) make sure that no OVERLAY enum
value is used in the core any longer because it gets mapped to
SOFTLIGHT at all entry points, with the downside of introducing a
setter/getter asymmetry when OVERLAY was set in a PDB api.