preserve sequence ID/start-end style switch for files read in to jalview

If Jalview is being used to quickly edit a file when working with a program, its very annoying to have to modify your preferences to prevent jalview from outputting the /start-end suffixes on the sequence ID.

1. Start-end suffixes could be a per-alignment setting inherited from the global preferences or set explicitly.
2. A third state 'preserve' could be provided which preserves the current start/end style.
.. so if a file is imported that does not include start-end suffix, then none is written out if it is exported again, and vice verca for files that include start-end suffixes.