Decorators were only added for the first item of an array.

Decorators for all items of an array are added:

• if the layout is flagged UI_ITEM_PROP_DECORATE automatically in uiItemFullR or
• calling uiItemDecoratorR (but only in certain situations, see below)

When calling uiItemDecoratorR with an index of 0, the following happens:

• the index is passed to uiItemDecoratorR_prop
• that checks with ui_item_rna_is_expand if decorators should be added to all items of an array
• the check fails (because it only permits RNA_NO_INDEX -- which is -1)

So two things we can do:

• remain using uiItemDecoratorR (that would require to pass an index of RNA_NO_INDEX -- a bad level include -- or -1)
• just use uiLayoutSetPropDecorate to flag the row properly

This patch does later.