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/meson.build

@@ -70,6 +70,7 @@ sway_sources = files(
 	'commands/no_focus.c',
 	'commands/nop.c',
 	'commands/output.c',
+	'commands/popup_during_fullscreen.c',
 	'commands/reload.c',
 	'commands/rename.c',
 	'commands/resize.c',