add logger for config errors and <core><errorCommand> to display it.

include/common/log.h

@@ -0,0 +1,21 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+#ifndef LABWC_LOG_H
+#define LABWC_LOG_H
+
+#include <wlr/util/log.h>
+#include "common/buf.h"
+
+struct buf *log_get_buf(void);
+void log_set_error(enum wlr_log_importance importance);
+void log_reset(void);
+void nag_show(void);
+void nag_show_callback(void *data);
+
+#define nag_log(verb, fmt, ...) \
+do { \
+	wlr_log(verb, fmt, ##__VA_ARGS__);						\
+	log_set_error(verb);									\
+	buf_add_fmt(log_get_buf(), fmt "\n", ##__VA_ARGS__);	\
+} while (0)
+
+#endif /* LABWC_LOG_H */