Implement scratchpad

Implements the following commands:

* move scratchpad
* scratchpad show
* [criteria] scratchpad show

Also fixes these:

* Fix memory leak when executing command with criteria
(use `list_free(views)` instead of `free(views)`)
* Fix crash when running `move to` with no further arguments

include/sway/scratchpad.h

@@ -0,0 +1,26 @@
+#ifndef _SWAY_SCRATCHPAD_H
+#define _SWAY_SCRATCHPAD_H
+
+#include "tree/container.h"
+
+/**
+ * Move a container to the scratchpad.
+ */
+void scratchpad_add_container(struct sway_container *con);
+
+/**
+ * Remove a container from the scratchpad.
+ */
+void scratchpad_remove_container(struct sway_container *con);
+
+/**
+ * Show or hide the next container on the scratchpad.
+ */
+void scratchpad_toggle_auto(void);
+
+/**
+ * Show or hide a specific container on the scratchpad.
+ */
+void scratchpad_toggle_container(struct sway_container *con);
+
+#endif

include/sway/server.h

@@ -48,6 +48,8 @@ struct sway_server {
 
 	list_t *transactions;
 	list_t *dirty_containers;
+
+	list_t *scratchpad; // struct sway_container
 };
 
 struct sway_server server;

include/sway/tree/container.h

@@ -135,6 +135,11 @@ struct sway_container {
 
 	struct sway_container *parent;
 
+	// Indicates that the container is a scratchpad container.
+	// Both hidden and visible scratchpad containers have scratchpad=true.
+	// Hidden scratchpad containers have a NULL parent.
+	bool scratchpad;
+
 	float alpha;
 
 	struct wlr_texture *title_focused;