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.

Source.h

@@ -148,14 +148,6 @@ public:
     // add callback to each update
     void call(SourceCallback *callback, bool override = false);
 
-    // callbacks associated to keys
-    void addInputCallback(uint input, SourceCallback *callback);
-    void removeInputCallback(SourceCallback *callback);
-    std::list<SourceCallback *> inputCallbacks(uint input);
-    std::list<uint> callbackInputs();
-    void swapInputCallback(uint from, uint to);
-    void clearInputCallbacks();
-
     // update mode
     inline  bool active () const { return active_; }
     virtual void setActive (bool on);
@@ -325,17 +317,6 @@ protected:
     // callbacks
     std::list<SourceCallback *> update_callbacks_;
     std::mutex access_callbacks_;
-    struct InputCallback {
-        bool active_;
-        SourceCallback *model_;
-        SourceCallback *reverse_;
-        InputCallback() {
-            active_ = false;
-            model_   = nullptr;
-            reverse_ = nullptr;
-        }
-    };
-    std::multimap<uint, InputCallback> input_callbacks_;
     void updateCallbacks(float dt);
 
     // clones