Blender Git Loki

UI: (Internal) utility for more controllable property split layout

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()`.