GeditView: override GtkTextView::create_buffer()

Before this patch, two buffers were created each time a new view is
created. Now only one buffer is created, the good one, GeditDocument.
GeditRecent: print a warning on failure

There was a problem when the mime type was not a valid UTF-8 string, and
it was quite difficult to debug. With a warning it is hopefully easier
to debug a future problem.