Blender Git Loki

Initial changes to checkbox layout: Use split layout & better section headings

See T65965.
Checkboxes now use the split layout as proposed in the design task and
columns can define headings now (`uiLayout.column(heading="Foo")`).
These headings are placed in the left column of the first item if that
doesn't add its own label there. Otherwise an own row is inserted for
the heading.