stylecontext: Revert part of previous commit
stylecontext: Revert part of previous commit

Commit 719dd636a9da679ea08a9644a8cccfe7d6f145d7 replaces
margin-left/right with margin-start/end. CSS does not have
margin-start/margin-end properties, the sed script was a bit overeager.

Fwiw, CSS implements RTL margin styling via :dir(rtl) selectors.

jessevdk/gtk,jessevdk/gtk,alexlarsson/gtk,alexlarsson/gtk,jigpu/gtk,chergert/gtk,Lyude/gtk-,Adamovskiy/gtk,grubersjoe/adwaita,grubersjoe/adwaita,ahodesuka/gtk,davidgumberg/gtk,Sidnioulz/SandboxGtk,chergert/gtk,davidgumberg/gtk,jessevdk/gtk,Lyude/gtk-,jadahl/gtk,alexlarsson/gtk,ahodesuka/gtk,davidgumberg/gtk,grubersjoe/adwaita,Lyude/gtk-,jadahl/gtk,Lyude/gtk-,davidgumberg/gtk,chergert/gtk,Sidnioulz/SandboxGtk,alexlarsson/gtk,jadahl/gtk,jessevdk/gtk,Adamovskiy/gtk,msteinert/gtk,chergert/gtk,chergert/gtk,davidgumberg/gtk,chergert/gtk,msteinert/gtk,Lyude/gtk-,ahodesuka/gtk,ahodesuka/gtk,alexlarsson/gtk,jadahl/gtk,ahodesuka/gtk,Adamovskiy/gtk,davidgumberg/gtk,jadahl/gtk,chergert/gtk,msteinert/gtk,jigpu/gtk,Sidnioulz/SandboxGtk,Adamovskiy/gtk,jessevdk/gtk,grubersjoe/adwaita,chergert/gtk,ahodesuka/gtk,jigpu/gtk,jadahl/gtk,ahodesuka/gtk,jigpu/gtk,msteinert/gtk,Sidnioulz/SandboxGtk,jessevdk/gtk,jigpu/gtk,jigpu/gtk,jadahl/gtk,ahodesuka/gtk,grubersjoe/adwaita,grubersjoe/adwaita,Sidnioulz/SandboxGtk,Adamovskiy/gtk,grubersjoe/adwaita,msteinert/gtk,Adamovskiy/gtk,jigpu/gtk,davidgumberg/gtk,Adamovskiy/gtk,davidgumberg/gtk,alexlarsson/gtk,Adamovskiy/gtk,Sidnioulz/SandboxGtk,jessevdk/gtk,jigpu/gtk,alexlarsson/gtk,jadahl/gtk,Lyude/gtk-,grubersjoe/adwaita,alexlarsson/gtk,msteinert/gtk,Lyude/gtk-,Lyude/gtk-