Blender Git Loki

UI: Keep track of button's relationship with label buttons

This is needed for property search where we want to highlight a button's label if it matches the search
filter. Otherwise the button itself is highlighted but it has no idea what label to highlight.

This is especially important because of the use of property split layouts where the label is almost
always outside of the button in a separate column.

The additional logic in the layout code isn't ideal, but I think this is generally a good thing to keep track of.
For example, another use for this could be adding a highlight to a buttons label on mouseover.

Differential Revision: https://developer.blender.org/D8783