implement the GET_LOG_MARKERS IPC request/reply

include/i3/ipc.h

@@ -40,6 +40,9 @@
 /** Request the configuration for a specific 'bar' */
 #define I3_IPC_MESSAGE_TYPE_GET_BAR_CONFIG      6
 
+/** Request the SHM debug log start/wrap markers */
+#define I3_IPC_MESSAGE_TYPE_GET_LOG_MARKERS     7
+
 /*
  * Messages from i3 to clients
  *
@@ -66,6 +69,9 @@
 /** Bar config reply type */
 #define I3_IPC_REPLY_TYPE_BAR_CONFIG            6
 
+/** Request the SHM debug log start/wrap markers */
+#define I3_IPC_REPLY_TYPE_LOG_MARKERS           7
+
 /*
  * Events from i3 to clients. Events have the first bit set high.
  *