Constify collect and lcopy strings in GTypeValueTable

This avoids warnings when creating idiomatic value tables, like:

  static const GTypeValueTable _clutter_shader_float_value_table = {
    clutter_value_init_shader_float,
    clutter_value_free_shader_float,
    clutter_value_copy_shader_float,
    clutter_value_peek_pointer,
    "ip",
    clutter_value_collect_shader_float,
    "pp",
    clutter_value_lcopy_shader_float
  };

Because the strings are literals. And, really: nobody should be using
allocated values for the collection and lcopy strings.

https://bugzilla.gnome.org/show_bug.cgi?id=671545