Previously `UILayout.prop_decorator()` (or `uiItemDecoratorR()` in C)
only worked for single items, not for arrays. The decorators are added
vertically, like `UILayout.prop()` adds them.
This will be needed for adding decorators to material properties, but
will likely have other use-cases as well.
Also, `None` (or `NULL`) can be passed for the data-pointer and property
now to create blank decorators (as already possible for
`uiItemDecoratorR_prop` in C).
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