clutter/evdev: disable mousekeys with Numlock ON

The clutter/evdev implementation of mousekeys is designed after the
current implementation in X11, and works when the setting is enabled
regardless of the status of NumLock.

The GNOME documentation on accessibility features states however that
mousekeys work only when NumLock is OFF:

  https://help.gnome.org/users/gnome-help/stable/mouse-mousekeys.html

Change the clutter/evdev implementation to match the documentation, i.e.
disable mousekeys when NumLock in ON so that switching NumLock ON
restores the numeric keypad behaviour.

Closes: https://gitlab.gnome.org/GNOME/mutter/issues/530
(cherry picked from commit 471b61bd14)

clutter/clutter/evdev/clutter-input-device-evdev.c

@@ -855,6 +855,14 @@ emulate_pointer_motion (ClutterInputDeviceEvdev *device,
   clutter_virtual_input_device_notify_relative_motion (device->mousekeys_virtual_device,
                                                        time_us, dx_motion, dy_motion);
 }
+static gboolean
+is_numlock_active (ClutterInputDeviceEvdev *device)
+{
+  ClutterSeatEvdev *seat = device->seat;
+  return xkb_state_mod_name_is_active (seat->xkb,
+                                       "Mod2",
+                                       XKB_STATE_MODS_LOCKED);
+}
 
 static void
 enable_mousekeys (ClutterInputDeviceEvdev *device)
@@ -1013,6 +1021,10 @@ handle_mousekeys_press (ClutterEvent            *event,
   if (!(event->key.flags & CLUTTER_EVENT_FLAG_SYNTHETIC))
     stop_mousekeys_move (device);
 
+  /* Do not handle mousekeys if NumLock is ON */
+  if (is_numlock_active (device))
+    return FALSE;
+
   /* Button selection */
   switch (event->key.keyval)
     {
@@ -1084,6 +1096,10 @@ static gboolean
 handle_mousekeys_release (ClutterEvent            *event,
                           ClutterInputDeviceEvdev *device)
 {
+  /* Do not handle mousekeys if NumLock is ON */
+  if (is_numlock_active (device))
+    return FALSE;
+
   switch (event->key.keyval)
     {
     case XKB_KEY_KP_0: