app: change GimpDrawable::convert_type() to take a target profile
instead of just a boolean "convert_profile". This takes the logic to figure the right target profile out of gimp_layer_convert_type(), it can't possibly know everything about how to convert anyway, and having the logic in the callers conveniently splits it up and distributes its parts to the places they belong. This commit should cause no behavor change and is just preparation for fixing bug 765176.
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