plug-ins: improve a bit PDF export progression.
Subdivise progression steps inside layer groups. It is not perfect yet as it would not handle perfectly weird edge cases where you have for instance a lot of layers in a top layer group with only many other top layers. But it would still handle it better than before. Ideally we should simply count all non-group "leaf" layers in the layer tree and use this as the number of steps. In other words, we should do a first pass through the whole layer tree.