Draft: popover: Reserve space for box-shadow

Marking as draft since it's not complete, there are some unsolved issues:

In GtkDropDown, the popover is not full width:

Not sure how to fix this one, since the dropdown allocates popover to a width matching itself. Maybe it could set the width of the list inside instead?

In GtkComboBox and also entry autocompletion the popover is offset as well:

(It has halign=fill as opposed to halign=start)

And noticed at the last second, but while it handles GtkPopover:position correctly, it doesn't account for when a popover changes position to avoid going out of bounds.

Fixes #1987 (closed)