lockscreen: Switch to layer-shell protocol

We use the overlay level for the lock screen so it's above 'top' which
is currently being used for the menus (until they turn into xdg popups).

Together with the input-inhibit protocol this make sure neither clients
nor other shell surfaces get any input.

src/phosh.c

@@ -239,12 +239,21 @@ lockscreen_create (PhoshShell *self)
   gdk_wayland_window_set_use_custom_surface (gdk_window);
 
   lockscreen->wl_surface = gdk_wayland_window_get_wl_surface (gdk_window);
-
-#if 0
-  phosh_mobile_shell_set_lock_surface(priv->mshell,
-                                      lockscreen->wl_surface);
-#endif
-  gtk_widget_show_all (lockscreen->window);
+  lockscreen->layer_surface = zwlr_layer_shell_v1_get_layer_surface(priv->layer_shell,
+                                                                    lockscreen->wl_surface,
+                                                                    priv->output,
+                                                                    ZWLR_LAYER_SHELL_V1_LAYER_OVERLAY,
+                                                                    "lockscreen");
+  zwlr_layer_surface_v1_set_exclusive_zone(lockscreen->layer_surface, -1);
+  zwlr_layer_surface_v1_set_size(lockscreen->layer_surface, 0, 0);
+  zwlr_layer_surface_v1_set_anchor(lockscreen->layer_surface,
+                                   ZWLR_LAYER_SURFACE_V1_ANCHOR_TOP |
+                                   ZWLR_LAYER_SURFACE_V1_ANCHOR_BOTTOM |
+                                   ZWLR_LAYER_SURFACE_V1_ANCHOR_LEFT |
+                                   ZWLR_LAYER_SURFACE_V1_ANCHOR_RIGHT);
+  zwlr_layer_surface_v1_set_keyboard_interactivity(lockscreen->layer_surface, TRUE);
+  zwlr_layer_surface_v1_add_listener(lockscreen->layer_surface, &layer_surface_listener, lockscreen);
+  wl_surface_commit(lockscreen->wl_surface);
   priv->lockscreen = lockscreen;
 
   priv->unlock_handler_id = g_signal_connect_swapped (