unified handling of timespans and time quantisation

[ichthyo]

The basic time handling is in place: we use gavl_time_t, together with a convenience wrapper for C++ (GUI and Proc). A bit more tricky is the question how to deal with time intervals, especially when having to consider frame quantisation.

• investigate the problems related to frame quantisation
• either find a suitable usage policy for gavl_time_t or write some C library functions
• augment lumiera::Time to return a time interval wrapper, which handles quantisation automatically (if this is possible, that is)

[ichthyo]

(In #84) Time wrapper type is now integrated and in use by Proc. Joel promised to adapt time handling in GUI too. Closing the main issue for now, and opening a new ticket for the remaining task of handling timespans and time quantisation

[ichthyo]

After discussion with Stefan Kangas, I'm grabbing this ticket now. Stefan continues to work on the Timecode display widget, while I'll concentrate on the logical side of defining Timespans and quantisation

[ichthyo]

basic framework for time handling, timecodes and quantisation is outlined now, yet still some important details need to be filled in