Introduce GET_BINDING_STATE IPC command

fixes #3892

testcases/t/311-get-binding-modes.t

@@ -0,0 +1,41 @@
+#!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 the GET_BINDING_MODE IPC command
+# Ticket: #3892
+# Bug still in: 4.18-318-g50160eb1
+use i3test i3_config => <<EOT;
+# i3 config file (v4)
+font -misc-fixed-medium-r-normal--13-120-75-75-C-70-iso10646-1
+
+mode "extra" {
+    bindsym Mod1+x nop foo
+}
+EOT
+
+my $i3 = i3(get_socket_path());
+$i3->connect->recv;
+# TODO: use the symbolic name for the command/reply type instead of the
+# numerical 12:
+my $binding_state = $i3->message(12, "")->recv;
+is($binding_state->{name}, 'default', 'at startup, binding mode is default');
+
+cmd 'mode extra';
+
+$binding_state = $i3->message(12, "")->recv;
+is($binding_state->{name}, 'extra', 'after switching, binding mode is extra');
+
+done_testing;