backup filenames: custom fields (timestamp, comment)

[FND]

Further pursuing the enhancements of #226 (changeset:2713), the attached patch allows for customized filenames in backups.

This includes parameters for a custom timestamp format, as well as an optional comment string (e.g. an edit summary) to be added to the filename:
original filename + timestamp + comment + extension

Note:
Currently, saveBackup() cannot simply be called on its own because of its dependence on localPath and original (both determined in saveChanges()).
It might be sensible to move those calculations into a separate function, then calling that function from within both saveChanges() and saveBackup() if necessary - thus enabling plugins/macros to simply call saveBackup() with the respective custom parameters at any time.

[JeremyRuston]

Concerned that this enhancement isn't of general enough utility. Perhaps we could refactor enough of the core to allow this functionality to be easily added as a plugin?