Implement a check that segment content is ENT_XML1 on saving

It must be ensured that our segment content is valid for export. That means: if a segment is saved, the entities & < > must be encoded as & etc. All other entities must not be encoded. On segment editing this is done automatically via the HTML editor. On other places like search and replace (see TRANSLATE-1691) the encoding must be ensured manually.

To prevent such errors again the validity of the segment content should be checked before saving a segment.