Commit d980e0ffee fixed an issue when the "Left" button would appear
at the right side for RTL languages given that GtkBox would reorder
its horizontal children to RTL direction.
The solution at the time was to force the primary-button box to stay
in LTR direction when the rest of the interface is shown in RTL.
This introduced a regression where the CSS "linked" style of the
buttons used was the LTR one, breaking the "linked" styling.
This fix will manually reorder the Left/Right buttons rather than
calling gtk_widget_set_direction on the primary-button-box.
Fixes#2649
358a952f70
