Blender Git Loki

Property Search: Build button groups for Search

This is needed for property search (upcoming patch D8856) where a
buttons labels are often separate buttons, but we need to highlight
the label to show that a property is a search result. This is
especially important in "property split" layouts where the label is
almost always another button in a separate column.

The button groups here are basically a flattened view of the buttons
in the layout tree. Every function that adds a new set of buttons
creates a new button group, and the new buttons are automatically
added to the most recent group. Then, each group is searched
separately in the property search phase. It's important that every
function adding a new button set calls layout_root_new_button_group.

Note that this won't be disabled when property search isn't active.
It may be useful for other things in the future, and trying to pass
that information to layout functions didn't feel worth it to me.

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