Introduce support for specifying variables from X resources. (#2286)

This patch introduces a new 'set_from_resource' config directive which
allows defining a variable by retrieving its value from the X resource
database. This avoids having to configure a color scheme in multiple
files. The directive takes an additional fallback value which is used
in case the resource cannot be found or during config validation where
no X connection is available.

Furthermore, this patch includes the following changes:
- If the same variable is defined twice, we now properly overwrite the
  value of the assignment rather than inserting two variable definitions
  with the same key.
- We now depend on xcb-util-xrm to query the resource.
- Increase the buffer size for variable / resource assignments.

fixes #2130

testcases/t/201-config-parser.t

@@ -441,7 +441,7 @@ client.focused          #4c7899 #285577 #ffffff #2e9ef4
 EOT
 
 my $expected_all_tokens = <<'EOT';
-ERROR: CONFIG: Expected one of these tokens: <end>, '#', 'set', 'bindsym', 'bindcode', 'bind', 'bar', 'font', 'mode', 'floating_minimum_size', 'floating_maximum_size', 'floating_modifier', 'default_orientation', 'workspace_layout', 'new_window', 'new_float', 'hide_edge_borders', 'for_window', 'assign', 'no_focus', 'focus_follows_mouse', 'mouse_warping', 'force_focus_wrapping', 'force_xinerama', 'force-xinerama', 'workspace_auto_back_and_forth', 'fake_outputs', 'fake-outputs', 'force_display_urgency_hint', 'focus_on_window_activation', 'show_marks', 'workspace', 'ipc_socket', 'ipc-socket', 'restart_state', 'popup_during_fullscreen', 'exec_always', 'exec', 'client.background', 'client.focused_inactive', 'client.focused', 'client.unfocused', 'client.urgent', 'client.placeholder'
+ERROR: CONFIG: Expected one of these tokens: <end>, '#', 'set', 'set_from_resource', 'bindsym', 'bindcode', 'bind', 'bar', 'font', 'mode', 'floating_minimum_size', 'floating_maximum_size', 'floating_modifier', 'default_orientation', 'workspace_layout', 'new_window', 'new_float', 'hide_edge_borders', 'for_window', 'assign', 'no_focus', 'focus_follows_mouse', 'mouse_warping', 'force_focus_wrapping', 'force_xinerama', 'force-xinerama', 'workspace_auto_back_and_forth', 'fake_outputs', 'fake-outputs', 'force_display_urgency_hint', 'focus_on_window_activation', 'show_marks', 'workspace', 'ipc_socket', 'ipc-socket', 'restart_state', 'popup_during_fullscreen', 'exec_always', 'exec', 'client.background', 'client.focused_inactive', 'client.focused', 'client.unfocused', 'client.urgent', 'client.placeholder'
 EOT
 
 my $expected_end = <<'EOT';

testcases/t/532-xresources.t

@@ -0,0 +1,64 @@
+#!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 using X resources in the config.
+# Ticket: #2130
+use i3test i3_autostart => 0;
+use X11::XCB qw(PROP_MODE_REPLACE);
+
+sub get_marks {
+    return i3(get_socket_path())->get_marks->recv;
+}
+
+my $config = <<EOT;
+# i3 config file (v4)
+font -misc-fixed-medium-r-normal--13-120-75-75-C-70-iso10646-1
+
+# This isn't necessarily what X resources are intended for, but it'll do the
+# job for the test.
+set_from_resource \$mark i3wm.mark none
+for_window [class=worksforme] mark \$mark
+
+set_from_resource \$othermark i3wm.doesnotexist none
+for_window [class=doesnotworkforme] mark \$othermark
+
+EOT
+
+$x->change_property(
+    PROP_MODE_REPLACE,
+    $x->get_root_window(),
+    $x->atom(name => 'RESOURCE_MANAGER')->id,
+    $x->atom(name => 'STRING')->id,
+    32,
+    length('*mark: works'),
+    '*mark: works');
+$x->flush;
+
+my $pid = launch_with_config($config);
+
+open_window(wm_class => 'worksforme');
+sync_with_i3;
+is_deeply(get_marks(), [ 'works' ], 'the resource has loaded correctly');
+
+cmd 'kill';
+
+open_window(wm_class => 'doesnotworkforme');
+sync_with_i3;
+is_deeply(get_marks(), [ 'none' ], 'the resource fallback was used');
+
+exit_gracefully($pid);
+
+done_testing;