Implement popup_during_fullscreen

This introduces a new view_impl function: is_transient_for. Similar to
container_has_ancestor but works using the surface parents rather than
the tree.

This patch modifies view_is_visible, container_at and so on to allow
transient views to function normally when they're in front of a
fullscreen view.

sway/commands.c

@@ -109,6 +109,7 @@ static struct cmd_handler handlers[] = {
 	{ "new_window", cmd_default_border },
 	{ "no_focus", cmd_no_focus },
 	{ "output", cmd_output },
+	{ "popup_during_fullscreen", cmd_popup_during_fullscreen },
 	{ "raise_floating", cmd_raise_floating },
 	{ "seat", cmd_seat },
 	{ "set", cmd_set },