Add support for Indent to simplify box statements with backgroundInsets

[nimakro]

The following code snippet:
treeTableCell { backgroundInsets += box(1.5.px, 0.0.px, 1.5.px, 0.0.px)}
should show an indent that the box statement could be simplified to:
treeTableCell { backgroundInsets += box(1.5.px, 0.0.px)}