Add "mode" field in binding event

This does *not* go in the binding object to reflect the same hierarchy
of the config file: a mode is a collection of bindings.

Fixes #5323

testcases/t/238-ipc-binding-event.t

@@ -16,6 +16,7 @@
 #
 # Test that the binding event works properly
 # Ticket: #1210
+# Ticket: #5323
 use i3test i3_autostart => 0;
 
 my $keysym = 't';
@@ -28,6 +29,12 @@ my $config = <<EOT;
 font -misc-fixed-medium-r-normal--13-120-75-75-C-70-iso10646-1
 
 bindsym $binding_symbol $command
+
+bindsym m mode some-mode
+mode some-mode {
+    bindsym a nop
+    bindsym b mode default
+}
 EOT
 
 SKIP: {
@@ -46,12 +53,13 @@ SKIP: {
 	    qx(xdotool key $binding_symbol);
 	},
 	'binding');
-
     is(scalar @events, 1, 'Received 1 event');
 
     is($events[0]->{change}, 'run',
         'the `change` field should indicate this binding has run');
 
+    is($events[0]->{mode}, 'default', 'the `mode` field should be "default"');
+
     ok($events[0]->{binding},
         'the `binding` field should be a hash that contains information about the binding');
 
@@ -69,6 +77,21 @@ SKIP: {
     is($events[0]->{binding}->{input_code}, 0,
         'the input_code should be the specified code if the key was bound with bindcode, and otherwise zero');
 
+    # Test that the mode field is correct
+    # See #5323.
+    @events = events_for(
+	sub {
+	    qx(xdotool key m);
+	    qx(xdotool key a);
+	    qx(xdotool key b);
+	},
+	'binding');
+    is(scalar @events, 3, 'Received 3 events');
+
+    is($events[0]->{mode}, 'default', 'Event for binding to enter new mode is atributed to default mode');
+    is($events[1]->{mode}, 'some-mode', 'Event for binding while in mode is attributed to the non-default mode');
+    is($events[2]->{mode}, 'some-mode', 'Event for binding exiting mode is attributed to the non-default mode');
+
     exit_gracefully($pid);
 
 }