transaction: use 'location' instead of 'vfs' in the addbackup method This unlock the backup of file outside of store (eg: bookmarks).

addbackup: handle file in subdirectory The current naming scheme ('journal.backups.<file>') resulted is bad directory name when 'file' was in a subdirectory. We now extract the directory name and create the backupfile within it. We plan to use file in a subdirectory for cachefile.

addbackup: use the vfs for the backup destination too The backup file location was always computed using the opener, bypassing the 'location' setting. (And making the feature broken.)