ipc-json,swaysavetree: round-trip placeholder swallows

ipc_json_describe_container emits the original swallows JSON for
unfilled placeholders. New sway-save-tree binary dumps a
workspace tiling tree as append_layout-compatible JSON,
synthesising swallows from app_id / class / instance.

meson.build

@@ -195,6 +195,7 @@ subdir('protocols')
 subdir('common')
 subdir('sway')
 subdir('swaymsg')
+subdir('swaysavetree')
 
 if get_option('swaybar') or get_option('swaynag')
 	subdir('client')