container: retain focus position on floating enable

When a tiling container is floated, the focus stack needs to be
appropraitely modified to return the container to its original
position in the tree upon floating disable, like i3.

sway/tree/container.c

@@ -821,6 +821,8 @@ void container_set_floating(struct sway_container *container, bool enable) {
 
 	struct sway_seat *seat = input_manager_current_seat();
 	struct sway_workspace *workspace = container->pending.workspace;
+	struct sway_container *focus = seat_get_focused_container(seat);
+	bool set_focus = focus == container;
 
 	if (enable) {
 		struct sway_container *old_parent = container->pending.parent;
@@ -835,6 +837,10 @@ void container_set_floating(struct sway_container *container, bool enable) {
 		container_floating_set_default_size(container);
 		container_floating_resize_and_center(container);
 		if (old_parent) {
+			if (set_focus) {
+				seat_set_raw_focus(seat, &old_parent->node);
+				seat_set_raw_focus(seat, &container->node);
+			}
 			container_reap_empty(old_parent);
 		}
 	} else {
@@ -846,7 +852,11 @@ void container_set_floating(struct sway_container *container, bool enable) {
 		struct sway_container *reference =
 			seat_get_focus_inactive_tiling(seat, workspace);
 		if (reference) {
-			container_add_sibling(reference, container, 1);
+			if (reference->view) {
+				container_add_sibling(reference, container, 1);
+			} else {
+				container_add_child(reference, container);
+			}
 			container->pending.width = reference->pending.width;
 			container->pending.height = reference->pending.height;
 		} else {