GUID not writable and unrecognized MgdSchema types since midgard core upgrade

[feri]

midgardmvc_ui_create_container tries to set the GUID of placeholder objects. This is now impossible since the new core (10.05-5.1) prevents such operation.

An other annoyance is that the latest midgard core brought in some changes which causes midgardmvc_ui_create_rdfmapper not to recognize mgd schema types.

Easy to reproduce the problems, just upgrade midgard from OBS (need to do it by hand; don't rely on apt) and then run any MVC app, like the new midgard-project.org site, or OpenKeidas.