consolidate engine-dialog widget sensitivity logic.

Use a single function with the complete logic.

Since the callgraph is complex, there is internal state as well as GUI
state (different pages), do not rely on individual methods to get it
right.

A widget's sensitivity should only be controlled by one function.

gtk2_ardour/engine_dialog.h

@@ -292,6 +292,7 @@ class EngineControl : public ArdourDialog, public PBD::ScopedConnectionList {
     void manage_control_app_sensitivity ();
     int push_state_to_backend (bool start);
     void post_push ();
+    void update_sensitivity ();
 
     /* latency measurement */
     void latency_button_clicked ();