Bug 774971 - Display errors outputted by GEGL.

Add a log handler so that GIMP can display errors outputted by GEGL.
Since third party code may run in threads and we have no control on
these, we have to be sure GTK+ code is run in a thread-safe way, hence
the usage of gdk_threads_add_idle_full(). This was the case here for
GEGL, and handling GEGL logs the same way as other GIMP logs would
result in crashes.