Implement the window::fullscreen_mode event

The fullscreen_mode event is a window with the "change" property set to
"fullscreen_mode". This event should be emitted whenever a window enters
or exits fullscreen mode.

This event can be used to turn off dpms off when a window is fullscreen
or display the fullscreen container name in the status line for
instance.

testcases/t/225-ipc-window-fullscreen.t

@@ -0,0 +1,59 @@
+#!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)
+#
+# Tests that the ipc window::fullscreen_mode event works properly
+#
+# Bug still in: 4.7.2-135-g7deb23c
+use i3test;
+
+my $i3 = i3(get_socket_path());
+$i3->connect()->recv;
+
+my $cv;
+my $t;
+
+sub reset_test {
+    $cv = AE::cv;
+    $t = AE::timer(0.5, 0, sub { $cv->send(0); });
+}
+
+reset_test;
+
+$i3->subscribe({
+        window => sub {
+            my ($e) = @_;
+            if ($e->{change} eq 'fullscreen_mode') {
+                $cv->send($e->{container});
+            }
+        },
+    })->recv;
+
+my $window = open_window;
+
+cmd 'fullscreen';
+my $con = $cv->recv;
+
+ok($con, 'got fullscreen window event (on)');
+is($con->{fullscreen_mode}, 1, 'window is fullscreen');
+
+reset_test;
+cmd 'fullscreen';
+$con = $cv->recv;
+
+ok($con, 'got fullscreen window event (off)');
+is($con->{fullscreen_mode}, 0, 'window is not fullscreen');
+
+done_testing;