Change indexes for FilterParameter from ASCII strings to numeric IDs

[azonenberg]

Having integer IDs starting at 0 would allow sorting parameters independently of lexical ordering, and also be much more i18n friendly.

[tarunik]

Sidenote: gtkmm's translation API is a wrapper around gettext() which uses the English string directly as a key, so I'm not sure how much this'd actually gain us re: i18n. Your point about parameter sorting is fair, though, although perhaps we could look at something more sophisticated yet if we wanted to encode grouping information?

[azonenberg]

Bumping this to 0.3 since it's going to be a pain to do without breaking existing scopesession's. We'll probably have to store the canonical English text in the file and use localized for display or something.

Maybe i18n can be the focus for 0.3, we'll figure it out.