Reimplementation of SourceInputCallbacks into Session

Session should be the object holding the list of inputs parameters (e.g. synchrony) and the list of source callbacks. This also avoids mixing input when copying sources.  Code could be improved but is operational.

SourceList.cpp

@@ -151,7 +151,10 @@ SourceList join (const SourceList &first, const SourceList &second)
     return l;
 }
 
-
+SourceLink::SourceLink(uint64_t id, Session *se): host_(nullptr), target_(nullptr), id_(0)
+{
+    connect(id, se);
+}
 
 SourceLink::SourceLink(Source *s): host_(nullptr), target_(nullptr), id_(0)
 {