Add g_seconds_to_time function

Adds a function that converts seconds to a time string like: 100 -> "1:40". This is a C port from the Granite Vala library seconds_to_time function here: https://github.com/elementary/granite/blob/master/lib/DateTime.vala#L183

I'm not sure if such function would go to glib/gdatetime.c or elsewhere so feedback is welcome.