Layers can now be duplicated
Small translation engine optimizations
When a layer is copied, automatically increment its name with a trailing number

e.g. "<base layer name> (2)".  The new support function that handles
this is generalized around any string, and it's smart enough to
automatically detect an existing number.  Thus a string like "<base
layer name> (2)" gets incremented to "<base layer name> (3)", as opposed
to "<base layer name> (2) (2)".

I like how GIMP calls duplicate layers "<base layer name> Copy", "<base
layer name> Copy 2", etc, but that approach has ugly translation
implications, so for now I'm just going with a numeric indicator when
duplicating layers.