add symbolic constant for "neutral" time.

Note: it's rather arbitrary choosen,
because any time grid can define its own origin.
You must not assume that the origin of any time axis
is located at Time::ZERO !!

src/lib/time/lumitime.cpp

@@ -39,6 +39,7 @@ namespace time {
   /** @note the allowed time range is explicitly limited to help overflow protection */
   const Time Time::MAX ( TimeValue::buildRaw_(+std::numeric_limits<gavl_time_t>::max() / 30) );
   const Time Time::MIN ( TimeValue::buildRaw_(-_raw(Time::MAX)                             ) );
+  const Time Time::ZERO;
   
   
   /** convenience constructor to build an

src/lib/time/timevalue.hpp

@@ -273,6 +273,7 @@ namespace time {
     public:
       static const Time MAX ; 
       static const Time MIN ;
+      static const Time ZERO;
       
       explicit 
       Time (TimeValue const& val =TimeValue(0))