backup & restore: user context (blocks and comments)

Should be including this in backup & restore?

Two possibilities:

• 1 dir per user, each dir with its own comments.xml, blocks subdir... (i.e. handling them like any other context)
• add that information to the users.xml file, so each user will have his comments, blocks... there

To consider, but keeping it lower priority.

Note: user files (icon/profile/private) are already being saved by backup_annotate_all_user_files()