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'.

testcases/t/205-ipc-windows.t

@@ -0,0 +1,43 @@
+#!perl
+# vim:ts=4:sw=4:expandtab
+#
+# Please read the following documents before working on tests:
+# • http://build.i3wm.org/docs/testsuite.html
+#   (or docs/testsuite)
+#
+# • http://build.i3wm.org/docs/lib-i3test.html
+#   (alternatively: perldoc ./testcases/lib/i3test.pm)
+#
+# • http://build.i3wm.org/docs/ipc.html
+#   (or docs/ipc)
+#
+# • http://onyxneon.com/books/modern_perl/modern_perl_a4.pdf
+#   (unless you are already familiar with Perl)
+
+use i3test;
+
+my $i3 = i3(get_socket_path());
+$i3->connect()->recv;
+
+################################
+# Window event
+################################
+
+# Events
+
+my $new = AnyEvent->condvar;
+$i3->subscribe({
+    window => sub {
+        my ($event) = @_;
+        $new->send($event->{change} eq 'new');
+    }
+})->recv;
+
+open_window;
+
+my $t;
+$t = AnyEvent->timer(after => 0.5, cb => sub { $new->send(0); });
+
+ok($new->recv, 'Window "new" event received');
+
+done_testing;