Fix pid lookup for the Kill action

Fix pid lookup for the Kill action

Before this patch, labwc would happily kill itself when the user
called the `Kill` action when any xwayland view had focus.

The reason this happened was that wlroots creates the xwayland
wayland client via socketpair() and thus a lookup of the pid
of the socket connection would return the pid of labwc itself.

This patch fixes that by implementing different pid lookup
mechanisms based on the view implementation backend.

Fixes: #1739