popover: Correct placement of popovers

popover: Correct placement of popovers

When we don't have a pointing-to rectangle, we want to place
the popover wrt to the parents bounds. But if we have a
pointing-to rectangle, it is relative to the widgets allocation,
which is different from the bounds.

We were not handling the second case correctly, leading to context
menus in the text view being mispositioned by the widgets CSS padding.

While we are touching this code, rewrite it to handle transforms.

Fixes: #5695