Export the old JCatEngine property to preserve internal ABI

[hughsie]

Fixes fwupd/fwupd#2223 (comment)

[superm1]

LGTM

[hughsie]

Hah, so I double checked for sanity:

<hughsie> ebassi, GLib question if i may. Is it okay to do something like:
<hughsie>  g_object_class_install_property (object_class, PROP_METHOD, g_param_spec_uint ("one", NULL, NULL, 0, 1, 0, G_PARAM_READWRITE));
<hughsie>  g_object_class_install_property (object_class, PROP_METHOD, g_param_spec_uint ("two", NULL, NULL, 0, 1, 0, G_PARAM_READABLE));
<ebassi> Nope
<ebassi> Unless you want to overwrite "one" with "two"