Adds a wrapper-struct to create and return the three layouts required for the propery split layout (i.e. `UILayout.use_property_split`). This gives more flexibility for special treatment. E.g. needed for adding the arrow icon buttons when there is a hierarchy of nodes to be represented in the material properties (needs inserting in the text column to not offset the split layout). This commit also makes use of the utility for `uiItemL_respect_property_split()`.