Replace hacky L_FLOATING container with a list

Workspaces previously had a magical `workspace->floating` container,
which had a layout of L_FLOATING and whose children were actual floating
views. This allowed some conveniences, but was a hacky solution because
the container has to be exempt from focus, coordinate transactions with
the workspace, and omit emitting IPC events (which we didn't do).

This commit changes it to be a list directly in the sway_workspace. The
L_FLOATING layout is no longer used so this has been removed as well.

* Fixes incorrect check in the swap command (it checked if the
containers had the L_FLOATING layout, but this layout applied to the
magical container).
* Introduces workspace_add_floating

sway/commands/floating.c

@@ -33,7 +33,7 @@ struct cmd_results *cmd_floating(int argc, char **argv) {
 	// If the container is in a floating split container,
 	// operate on the split container instead of the child.
 	if (container_is_floating_or_child(container)) {
-		while (container->parent->layout != L_FLOATING) {
+		while (container->parent->type != C_WORKSPACE) {
 			container = container->parent;
 		}
 	}

sway/commands/focus.c

@@ -39,7 +39,7 @@ static struct cmd_results *focus_mode(struct sway_container *con,
 	// If the container is in a floating split container,
 	// operate on the split container instead of the child.
 	if (container_is_floating_or_child(con)) {
-		while (con->parent->layout != L_FLOATING) {
+		while (con->parent->type != C_WORKSPACE) {
 			con = con->parent;
 		}
 	}

sway/commands/move.c

@@ -449,7 +449,7 @@ static struct cmd_results *move_to_scratchpad(struct sway_container *con) {
 	// If the container is in a floating split container,
 	// operate on the split container instead of the child.
 	if (container_is_floating_or_child(con)) {
-		while (con->parent->layout != L_FLOATING) {
+		while (con->parent->type != C_WORKSPACE) {
 			con = con->parent;
 		}
 	}

sway/commands/scratchpad.c

@@ -16,7 +16,7 @@ static void scratchpad_toggle_auto(void) {
 	// If the focus is in a floating split container,
 	// operate on the split container instead of the child.
 	if (container_is_floating_or_child(focus)) {
-		while (focus->parent->layout != L_FLOATING) {
+		while (focus->parent->type != C_WORKSPACE) {
 			focus = focus->parent;
 		}
 	}
@@ -33,9 +33,8 @@ static void scratchpad_toggle_auto(void) {
 
 	// Check if there is an unfocused scratchpad window on the current workspace
 	// and focus it.
-	for (int i = 0; i < ws->sway_workspace->floating->children->length; ++i) {
-		struct sway_container *floater =
-			ws->sway_workspace->floating->children->items[i];
+	for (int i = 0; i < ws->sway_workspace->floating->length; ++i) {
+		struct sway_container *floater = ws->sway_workspace->floating->items[i];
 		if (floater->scratchpad && focus != floater) {
 			wlr_log(WLR_DEBUG,
 					"Focusing other scratchpad window (%s) in this workspace",
@@ -103,7 +102,7 @@ struct cmd_results *cmd_scratchpad(int argc, char **argv) {
 		// If the container is in a floating split container,
 		// operate on the split container instead of the child.
 		if (container_is_floating_or_child(con)) {
-			while (con->parent->layout != L_FLOATING) {
+			while (con->parent->type != C_WORKSPACE) {
 				con = con->parent;
 			}
 		}

sway/commands/swap.c

@@ -72,7 +72,7 @@ struct cmd_results *cmd_swap(int argc, char **argv) {
 			|| container_has_ancestor(other, current)) {
 		error = cmd_results_new(CMD_FAILURE, "swap",
 				"Cannot swap ancestor and descendant");
-	} else if (current->layout == L_FLOATING || other->layout == L_FLOATING) {
+	} else if (container_is_floating(current) || container_is_floating(other)) {
 		error = cmd_results_new(CMD_FAILURE, "swap",
 				"Swapping with floating containers is not supported");
 	}