Create a backup when saving projects (and other files?)

Jalview should always (or by default) create a backup of any file that is overwritten due to a save operation. In most cases, Jalview will prompt the user to ask whether they wish to overwrite an existing file, except in the case when the user hits CTRL/CMD+S - which simply overwrites the file where the data was first loaded from.

Implementing this is relatively trivial, but we ideally also need sensible naming scheme for the backups that allows someone to easily recover previous versions. See comments for suggestions.