[Azoth Xoox] Slight refactoring and code style fixes.
[Azoth Xoox] RoomParticipantEntry::GetPhotoHash() for #1675.
[Azoth Xoox] Always recreate the entry on renames.
[Azoth Xoox] Moved message merging to MergeMessages().
[Azoth Xoox] Merge & emit other entry's unread messages on rename.

Finally fixes #1675.
[Azoth Xoox] Use inplace merge instead.