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This commit just changes our saving API (i.e. the GimpSaveProcedure
class) to take an array of drawables as argument instead of a single
drawable.

It actually doesn't matter much for exporting as the whole API seems
more or less bogus there and all formats plug-ins mostly care only
whether they will merge/flatten all visible layers (the selected ones
don't really matter) or if the format supports layers of some sort. It
may be worth later strengthening a bit this whole logics, and maybe
allow partial exports for instance.

As for saving, it was not even looking at the passed GimpDrawable either
and was simply re-querying the active layer anyway.
Note that I don't implement the multi-selection saving in XCF yet in
this commit. I only updated the API. The reason is that the current
commit won't be backportable to gimp-2-10 because it is an API break. On
the other hand, the code to save multi-selection can still be backported
even though the save() API will only pass a single drawable (as I said
anyway, this argument was mostly bogus until now, hence it doesn't
matter much for 2.10 logics).