src/action.c: Convert action->arg to a list of action_arg

In preperation for Actions that require multiple arguments

include/private/action.h

@@ -0,0 +1,24 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+#ifndef __LABWC_PRIVATE_ACTION_H
+#define __LABWC_PRIVATE_ACTION_H
+
+/* Don't include ourself as search path starts at current directory */
+#include "../action.h"
+
+enum action_arg_type {
+	LAB_ACTION_ARG_STR = 0,
+};
+
+struct action_arg {
+	struct wl_list link;        /* struct action.args */
+
+	const char *key;            /* May be NULL if there is just one arg */
+	enum action_arg_type type;
+};
+
+struct action_arg_str {
+	struct action_arg base;
+	char *value;
+};
+
+#endif /* __LABWC_PRIVATE_ACTION_H */