Improved management of selection in Player

Mixer.h

@@ -86,6 +86,7 @@ public:
     Source * findSource (std::string name);
     Source * findSource (uint64_t id);
     SourceList findSources (float depth_from, float depth_to);
+    SourceList validate(const SourceList &list);
 
     // management of view
     View *view   (View::Mode m = View::INVALID);