openraster: Implements exporting and loading group layers.

This preserves the layer structure and
brings it in pair to MyPaint development branch
(allowing interchange of .ora files with group layers).

Previous behavior on load was oppening all
sub-layers in a flat-space, with no groups,
and flatten layer groups on export.

The implemented behavior accords to the
openraster spec.

Conflicts:
	plug-ins/pygimp/plug-ins/file-openraster.py