Update with GLib 2.78

If I set glib_req_version to 2.78 in top-level meson.build, I get some warnings.

libplanner/mrp-storage-module-factory.c:113:9:
warning: ‘g_module_build_path’ is deprecated [-Wdeprecated-declarations]
  113 |         libname = g_module_build_path (path, fullname);

warning: ignoring return value of ‘g_string_free_and_steal’
libplanner/mrp-task-manager.c:524:9: note: in expansion of macro ‘g_string_free’
  524 |         g_string_free (str, FALSE);

src/libgnomecanvas/gnome-canvas-pixbuf.c:95:9: 
warning: ‘g_type_class_add_private’ is deprecated
   95 |         g_type_class_add_private (class, sizeof (GnomeCanvasPixbufPrivate));

src/libgnomecanvas/gnome-canvas-pixbuf.c:102:20: 
warning: Deprecated pre-processor symbol: replace with "G_ADD_PRIVATE"
  102 |         gcp->priv = GNOME_CANVAS_PIXBUF_GET_PRIVATE (gcp);