Support _NET_WM_STATE_FOCUSED

_NET_WM_STATE_FOCUSED is set on _NET_WM_STATE to indicate that the
window is focused. It must be set when the window is newly focused and
removed once the window no longer has focus.

> _NET_WM_STATE_FOCUSED indicates whether the window's decorations are
> drawn in an active state. Clients MUST regard it as a read-only hint.
> It cannot be set at map time or changed via a _NET_WM_STATE client
> message.

For example, this is used by GTK applications to show the decoration in
an active or inactive state. This change can be tested by opening a GTK
application (like evince), focusing the window and unfocusing the
window, and observing a change in the window decorations.

Fixes #2273

testcases/t/295-net-wm-state-focused.t

@@ -0,0 +1,37 @@
+#!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 for setting and removing the _NET_WM_STATE_FOCUSED atom properly.
+# Ticket: #2273
+use i3test;
+use X11::XCB qw(:all);
+
+my ($windowA, $windowB);
+
+fresh_workspace;
+$windowA = open_window;
+
+ok(is_net_wm_state_focused($windowA), 'a newly opened window that is focused should have _NET_WM_STATE_FOCUSED set');
+
+$windowB = open_window;
+
+ok(!is_net_wm_state_focused($windowA), 'when a another window is focused, the old window should not have _NET_WM_STATE_FOCUSED set');
+
+fresh_workspace;
+
+ok(!is_net_wm_state_focused($windowB), 'when focus moves to the ewmh support window, neither window should have _NET_WM_STATE_FOCUSED set');
+
+done_testing;