Make channels a product type instead of subtype.

Now we can provide different channel expressions for one particular channel build. Not sure yet how this would be useful, but I found it more appropriate to use a type instead of a subtype of "file".

This should get us consistent with the provious commit.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>

Created by  aszlig  on April 20, 2015
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