popover: Don't inherit style from parent

popover: Don't inherit style from parent

Instead, inherit style from toplevel (because that's the default way,
not because it makes lots of sense).

This way, popovers don't inherit the styling from the widget that popped
them up, which is a problem in selected listbox rows, selection-mode
headerbars.
It also doesn't inherit styling where we might want it, like the osd.

But we can only have one of the two things.