ScriptFu: Plugins: Decor>Stencil Chrome revamp

Some changes to ScriptFu itself, re file args.

A thorough revamp of Stencil Chrome, a ScriptFu plugin.

No intended change in the effect, except it now will work on an image of any mode, with or without alpha. When a grayscale image without alpha (the former requirement) is the source image, should be exactly the same.

Use v3 binding, to test that feature of ScriptFu once more.

Some styling changes and added comments.

The description should also be changed, but not during string-freeze.

Compare to Stencil Carve, very similar, which also could use a revamp.