popover: Support shadows

popover: Support shadows

Use gdk_popup_layout_set_shadow_width to take shadows into
account when positioning popovers, and set the input region
to exclude the shadow, since we aren't doing interactive
resizing and the like.

When the popover has a beak, we make the surface size be
content size + shadow + tail, and then position the content
according to the final position inside this slightly too large
surface. The surface being too large doesn't matter, since we
set up an input region.