Use `purple_str_seconds_to_string' function instead of `last_seen_text'.

Use the `purple_str_seconds_to_string` function provided by `libpurple` to convert seconds to a string.  This is much more fine-grained than `last_seen_text` and has the advantage to return a localized string.

I replaced the `Last seen` string with `Last seen on WhatsApp` to avoid confusion with the `Last seen` string provided by the `lastseen` plugin, part of the [Purple plugin pack](https://bitbucket.org/rekkanoryo/purple-plugin-pack).  I hope this isn't a problem.