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?