introduce the sync IPC command

Sending the sync command via IPC ensures pending IPC messages are handled by i3
before the sync response is read. This is rarely useful for direct IPC
connections to i3, but becomes useful when synchronizing with i3bar, which might
have pending IPC messages in response to button clicks.

include/i3/ipc.h

@@ -63,6 +63,9 @@ typedef struct i3_ipc_header {
 /** Send a tick event to all subscribers. */
 #define I3_IPC_MESSAGE_TYPE_SEND_TICK 10
 
+/** Trigger an i3 sync protocol message via IPC. */
+#define I3_IPC_MESSAGE_TYPE_SYNC 11
+
 /*
  * Messages from i3 to clients
  *
@@ -78,6 +81,7 @@ typedef struct i3_ipc_header {
 #define I3_IPC_REPLY_TYPE_BINDING_MODES 8
 #define I3_IPC_REPLY_TYPE_CONFIG 9
 #define I3_IPC_REPLY_TYPE_TICK 10
+#define I3_IPC_REPLY_TYPE_SYNC 11
 
 /*
  * Events from i3 to clients. Events have the first bit set high.