Handle _NET_CLOSE_WINDOW client message requests

> Pagers wanting to close a window MUST send a _NET_CLOSE_WINDOW client
> message request to the root window.

We interpret this message as a request to close the con for the given
window.

See: http://standards.freedesktop.org/wm-spec/wm-spec-latest.html#idm140200472668896

fixes #1396

testcases/t/239-net-close-window-request.t

@@ -0,0 +1,49 @@
+#!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)
+#
+# Test _NET_CLOSE_WINDOW requests to close a window.
+# See http://standards.freedesktop.org/wm-spec/wm-spec-latest.html#idm140200472668896
+# Ticket: #1396
+# Bug still in: 4.8-116-gbb1f857
+use i3test;
+
+sub send_close_window_request {
+    my ($win) = @_;
+
+    my $msg = pack "CCSLLLLLL",
+        X11::XCB::CLIENT_MESSAGE, # response_type
+        32, # format
+        0, # sequence
+        $win->id, # window
+        $x->atom(name => '_NET_CLOSE_WINDOW')->id, # message type
+        0, # data32[0]
+        0, # data32[1]
+        0, # data32[2]
+        0, # data32[3]
+        0; # data32[4]
+
+    $x->send_event(0, $x->get_root_window(), X11::XCB::EVENT_MASK_SUBSTRUCTURE_REDIRECT, $msg);
+}
+
+my $ws = fresh_workspace;
+my $win = open_window;
+
+send_close_window_request($win);
+sync_with_i3;
+
+is(@{get_ws($ws)->{nodes}}, 0, 'When a pager sends a _NET_CLOSE_WINDOW request for a window, the container should be closed');
+
+done_testing;