fixed EXT-5526 Objects are shown as persons in the chat history from previous sessions

- new chat source type CHAT_SOURCE_UNKNOWN for avatar names not in cache and object names consisting of two words (avatar names like)
- new icon (unknown_icon.png) for message headers () of nearby chat messagses coming from CHAT_SOURCE_UNKNOWN
- context menu no longer displayed for object names from chat history (no UUIDs)
- double click on message headers in nearby chat no longer opens Inspector (no UUIDs)
- object names (loaded from chat history) in nearby chat are no longer SLURLed (no UUIDs for them)

--HG--
branch : product-engine