Make ClockTime a Option<u64> to handle GST_CLOCK_TIME_NONE more nicely
Currently we use u64::MAX for "no time known/set", just like in C. In C doing this caused various bugs when code started doing calculations with that value. We should try to do better here, which requires changes to https://github.com/gtk-rs/gir/ to allow manually creating bindings for ClockTime (and not make it a simple alias), and having FromGlib / ToGlib be implemented and called for it.