Blender Git Loki

UI: support drawing booleans with icons as check-boxes

Previously, if a boolean happened to use an icon there was no way
to make it display as a check-box from Python scripts.

The previous logic meant we ended up having to edit the RNA.
Since booleans with icons don't work well with the split-property layout
(now used for most of the interface).
Icons were being removed from RNA then added back using awkward Python
ternary expressions in the interface scripts.

The toggle argument now has an unset state (-1).

- toggle=True: no checkbox (emboss).
- toggle=False: always use a checkbox (no icon).
- toggle=(unset/-1): depends on the icon status, default as before.

Since toggle=False was default, this isn't used in existing UI logic.