GtkPopover: Ensure we unset modality before unparenting the popover

GtkPopover: Ensure we unset modality before unparenting the popover

Otherwise the gtk_grab_remove() calls on widget destruction will happen
on the default window group, which may leave the real window group
of the popover with a dangling pointer if it is not the default one.

This could be seen on the inspector, open a popover in the properties
list and close the window with alt-F4.