Add a new IPC event for changes on windows.

Added new event id (I3_IPC_EVENT_WINDOW) so that a an IPC client can
subscribe to events on windows. Added a basic window event that gets
triggered when a window gets successfully reparented. This new event
also dumps the container data, so that IPC clients can get the initial
window name. IPC clients wishing to see window events should subscribe
to 'window'.

docs/ipc

@@ -623,6 +623,9 @@ output (1)::
 	outputs, CRTCs or output properties).
 mode (2)::
 	Sent whenever i3 changes its binding mode.
+window (3)::
+	Sent when a client's window is successfully reparented (that is when i3
+	has finished fitting it into a container).
 
 *Example:*
 --------------------------------------------------------------------
@@ -696,6 +699,30 @@ mode is simply named default.
 { "change": "default" }
 ---------------------------
 
+=== window event
+
+This event consists of a single serialized map containing a property
++change (string)+ which currently can indicate only that a new window
+has been successfully reparented (the value will be "new").
+
+Additionally a +container (object)+ field will be present, which consists
+of the window's parent container. Be aware that the container will hold
+the initial name of the newly reparented window (e.g. if you run urxvt
+with a shell that changes the title, you will still at this point get the
+window title as "urxvt").
+
+*Example:*
+---------------------------
+{
+ "change": "new",
+ "container": {
+  "id": 35569536,
+  "type": 2,
+  ...
+ }
+}
+---------------------------
+
 == See also (existing libraries)
 
 [[libraries]]