This implied a lot of other core changes, which also pushed me into
improving some of the edit actions and PDB calls to be multi-layer aware
in the same time.

Note that it is still work-in-progress, but I just had to commit
something in an acceptable intermediate state otherwise I was just going
crazy.

In particular now the various transform tools are multi-layer aware and
work simultaneously on all selected layers (and the linked layers if any
of the selected layers is linked too). Both preview and final transform
processing works.
In the limitations, preview doesn't work well (only one layer in the
preview) when there is a selection (though the actual transform works).

Also I am left to wonder how we should process this case of canvas
selection+transform on multi-layers. Indeed currently I am just creating
a floating selection (like we used to for the selection+transform case)
containing a transform result of the composited version of all selected
layers. This is a possible expected result, but another could be to get
several transformed layers (without composition). But then should the
"Floating Selection" concept allow for multiple Floating Selections?
Sooo many questions left to answer.