move SafeCountable functionality to Editor from Window

src/editor.hh

@@ -17,7 +17,7 @@ class Register;
 //
 // The Editor class provides methods to manipulate a set of selections
 // and to use these selections to mutate it's buffer.
-class Editor
+class Editor : public SafeCountable
 {
 public:
     typedef std::function<SelectionAndCaptures (const Selection&)> Selector;

src/window.hh

@@ -19,7 +19,7 @@ class HighlighterGroup;
 // to the editing ones already provided by the Editor class.
 // Display can be customized through the use of highlighters handled by
 // the window's HighlighterGroup
-class Window : public Editor, public OptionManagerWatcher, public SafeCountable
+class Window : public Editor, public OptionManagerWatcher
 {
 public:
     ~Window();