gtkpopover: Fix allocation of child widget wrt margins
gtkpopover: Fix allocation of child widget wrt margins

During size request, all sides' margins are ensured to be as big
as TAIL_HEIGHT, just to avoid possible relocation loops if the
popover doesn't fit in its original position. This must be
accounted for in size_allocate() as well.

Lyude/gtk-,alexlarsson/gtk,alexlarsson/gtk,Adamovskiy/gtk,davidgumberg/gtk,jigpu/gtk,jigpu/gtk,chergert/gtk,Lyude/gtk-,Lyude/gtk-,alexlarsson/gtk,Adamovskiy/gtk,grubersjoe/adwaita,jigpu/gtk,ahodesuka/gtk,alexlarsson/gtk,grubersjoe/adwaita,jigpu/gtk,jadahl/gtk,davidgumberg/gtk,Adamovskiy/gtk,grubersjoe/adwaita,Lyude/gtk-,Lyude/gtk-,jadahl/gtk,ahodesuka/gtk,ahodesuka/gtk,Adamovskiy/gtk,jigpu/gtk,alexlarsson/gtk,chergert/gtk,ahodesuka/gtk,grubersjoe/adwaita,Lyude/gtk-,chergert/gtk,Adamovskiy/gtk,grubersjoe/adwaita,grubersjoe/adwaita,jigpu/gtk,davidgumberg/gtk,alexlarsson/gtk,Adamovskiy/gtk,ahodesuka/gtk,ahodesuka/gtk,Lyude/gtk-,chergert/gtk,Adamovskiy/gtk,alexlarsson/gtk,davidgumberg/gtk,chergert/gtk,ahodesuka/gtk,jadahl/gtk,chergert/gtk,jigpu/gtk,jigpu/gtk,davidgumberg/gtk,jadahl/gtk,jadahl/gtk,jadahl/gtk,alexlarsson/gtk,jadahl/gtk,chergert/gtk,jadahl/gtk,grubersjoe/adwaita,grubersjoe/adwaita,davidgumberg/gtk,chergert/gtk,Adamovskiy/gtk,Lyude/gtk-,ahodesuka/gtk,davidgumberg/gtk,davidgumberg/gtk