added the ability to save history for each buffer
Sessions now store the navigation history of each buffer, rather
than just the URI of the current page. In addition to history,
the title of each page is now saved alongside the URI.
The new session format is incompatible with the old one, but this
module will still read session files in the old format. It is
also possible to save a session in the old format using the command
session-save-old. This command may be removed in the future and
replaced with a more general mechanism to import and export session
files in other formats.