placessidebar: Point to row instead or reparenting popovers
In GTK 3 we used to move the popovers around using set_relative_to(); this is gone in GTK 4 and the apparent direct replacement is setting the target widget as the new parent. But this requires a lot of careful handling least the popover become orphan, which gets us ready to crash at any moment. Since we only care about positioning the popovers relative to a row, let's use the set_pointing_to() instead of reparenting. Now, the sidebar is always the parent.