EditorComponent should have a reference, not pointer, to an Editor

the value is not allow to be null, and the coding style guide says in such cases to prefer
a reference.

No semantics change here, just the fact that all EditorComponents receive *this instead
of this when the Editor that owns them creates them

gtk2_ardour/editor_component.h

@@ -23,10 +23,10 @@ class Editor;
 class EditorComponent
 {
 public:
-	EditorComponent (Editor *);
+	EditorComponent (Editor&);
 	virtual ~EditorComponent() {}
 
 protected:
-	Editor* _editor;
+	Editor& _editor;
 };