Handle WM_CLASS changes

http://tronche.com/gui/x/icccm/sec-4.html

> The WM_CLASS property (of type STRING without control characters)
> contains two consecutive null-terminated strings. These specify the
> Instance and Class names to be used by both the client and the window
> manager for looking up resources for the application or as identifying
> information.

i3 processes changes to WM_CLASS by updating the cached property and
running assignments. This allows the property to be used in criteria
selection.

fixes #1052

testcases/t/235-wm-class-change-handler.t

@@ -0,0 +1,70 @@
+#!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 that changes to WM_CLASS are internally processed by i3 by updating the
+# cached property and running assignments. This allows the property to be used
+# in criteria selection
+# Ticket: #1052
+# Bug still in: 4.8-73-g6bf7f8e
+use i3test i3_autostart => 0;
+use X11::XCB qw(PROP_MODE_REPLACE);
+
+my $config = <<EOT;
+# i3 config file (v4)
+font -misc-fixed-medium-r-normal--13-120-75-75-C-70-iso10646-1
+for_window [class="Special"] mark special_class_mark
+EOT
+
+my $pid = launch_with_config($config);
+
+sub change_window_class {
+    my ($window, $class) = @_;
+    my $atomname = $x->atom(name => 'WM_CLASS');
+    my $atomtype = $x->atom(name => 'STRING');
+    $x->change_property(
+        PROP_MODE_REPLACE,
+        $window->id,
+        $atomname->id,
+        $atomtype->id,
+        8,
+        length($class) + 1,
+        $class
+    );
+    sync_with_i3;
+}
+
+my $ws = fresh_workspace;
+
+my $win = open_window;
+
+change_window_class($win, "special\0Special");
+
+my $con = @{get_ws_content($ws)}[0];
+
+is($con->{window_properties}->{class}, 'Special',
+    'The container class should be updated when a window changes class');
+
+is($con->{window_properties}->{instance}, 'special',
+    'The container instance should be updated when a window changes instance');
+
+# The mark `special_class_mark` is added in a `for_window` assignment in the
+# config for testing purposes
+is($con->{mark}, 'special_class_mark',
+    'A `for_window` assignment should run for a match when the window changes class');
+
+exit_gracefully($pid);
+
+done_testing;