Refactor arrange_windows()

Replaces arrange_windows() with arrange_root(), arrange_output(),
arrange_workspace() and arrange_children_of().

Also makes fullscreen views save and restore their dimensions, which
allows it to preserve any custom resize and is also a requirement for
floating views once they are implemented.

include/sway/tree/container.h

@@ -75,6 +75,7 @@ struct sway_container {
 	double x, y;
 	// does not include borders or gaps.
 	double width, height;
+	double saved_width, saved_height;
 
 	list_t *children;