Implement a list/array data widget

Operation inputs can take fixed values (on the pipeline edit page: click on the operation input, then on the "Assign fixed value" icon) and there are widgets for some of the important base formats (strings, numbers, booleans) - however, it's not currently possible to edit structured values. The architecture of the data widget mechanism should allow nested widgets, so a value with schema format "php:type:array" could host other widgets for child elements of any format.

It might be a bit of a challenge to implement this properly, but for starters, the widget could decide only to generate arrays but not edit generic structures. (This is done by not returning an appropriate widget when the $data parameter of hook_transformations_data_widgets() is set.) Any extensions, say, "Add new item", naming keys, returning objects instead of arrays, and finally generic structure editing, can always be done at a later time :]