Blender Git Loki

UI: Add padding to the left of tree-rows labels without icon

The label was placed right at the left border of the row highlight,
which looked weird. So add some padding to tree-row labels without icon
or collapse chevron, which makes it look more polished. As additional
benefit, it alignes the labels better with icons of other rows on the
same tree level. And the padding makes it more clear that a child is
indeed a child, not just a sibling without icon.