Implement showing window icons in titlebar (#4439)

This feature defaults to off, and can be turned on for individual windows,
or (with for_window) for all new windows. See the userguide change.

This commit is partially based on work by:

• Marius Muja
• mickael9
• Esteve Varela Colominas
• Bernardo Menicagli

testcases/t/116-nestedcons.t

@@ -73,6 +73,7 @@ my $expected = {
     workspace_layout => 'default',
     current_border_width => -1,
     marks => $ignore,
+    window_icon_padding => -1,
 };
 
 # a shallow copy is sufficient, since we only ignore values at the root

testcases/t/187-commands-parser.t

@@ -176,6 +176,7 @@ is(parser_calls('unknown_literal'),
        scratchpad
        swap
        title_format
+       title_window_icon
        mode
        bar
     )) . "'\n" .

testcases/t/314-window-icon-padding.t

@@ -0,0 +1,62 @@
+#!perl
+# vim:ts=4:sw=4:expandtab
+#
+# Please read the following documents before working on tests:
+# • https://build.i3wm.org/docs/testsuite.html
+#   (or docs/testsuite)
+#
+# • https://build.i3wm.org/docs/lib-i3test.html
+#   (alternatively: perldoc ./testcases/lib/i3test.pm)
+#
+# • https://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)
+#
+# Verifies title_window_icon behavior.
+use i3test i3_autostart => 0;
+
+sub window_icon_padding {
+    my ($ws) = @_;
+    my ($nodes, $focus) = get_ws_content($ws);
+    ok(@{$nodes} == 1, 'precisely one container on workspace');
+    return $nodes->[0]->{'window_icon_padding'};
+}
+
+my $config = <<"EOT";
+# i3 config file (v4)
+font -misc-fixed-medium-r-normal--13-120-75-75-C-70-iso10646-1
+EOT
+my $pid = launch_with_config($config);
+
+my $tmp = fresh_workspace;
+
+cmd 'open';
+is(window_icon_padding($tmp), -1, 'window_icon_padding defaults to -1');
+
+cmd 'title_window_icon on';
+isnt(window_icon_padding($tmp), -1, 'window_icon_padding no longer -1');
+
+exit_gracefully($pid);
+
+################################################################################
+# Verify title_window_icon can be used with for_window as expected
+################################################################################
+
+$config = <<"EOT";
+# i3 config file (v4)
+font -misc-fixed-medium-r-normal--13-120-75-75-C-70-iso10646-1
+
+for_window [class=".*"] title_window_icon padding 3px
+EOT
+$pid = launch_with_config($config);
+
+$tmp = fresh_workspace;
+
+open_window;
+is(window_icon_padding($tmp), 3, 'window_icon_padding set to 3');
+
+exit_gracefully($pid);
+
+done_testing;