various small code cleanups and improvements

Largely refactoring work to make the code somewhat nicer to work with.

app/core.h

@@ -30,6 +30,7 @@
 #include "window/mainwindow/mainwindow.h"
 #include "task/task.h"
 #include "tool/tool.h"
+#include "undo/undostack.h"
 
 /**
  * @brief The main central Olive application instance
@@ -51,6 +52,13 @@ public:
    */
   Core();
 
+  /**
+   * @brief Core object accessible from anywhere in the code
+   *
+   * Use this to access Core functions.
+   */
+  static Core* instance();
+
   /**
    * @brief Start Olive Core
    *
@@ -74,6 +82,11 @@ public:
    */
   MainWindow* main_window();
 
+  /**
+   * @brief Retrieve UndoStack object
+   */
+  UndoStack* undo_stack();
+
   /**
    * @brief Import a list of files
    *
@@ -290,18 +303,19 @@ private:
    */
   QTimer autorecovery_timer_;
 
+  /**
+   * @brief Application-wide undo stack instance
+   */
+  UndoStack undo_stack_;
+
+  /**
+   * @brief Static singleton core instance
+   */
+  static Core instance_;
+
 private slots:
   void SaveAutorecovery();
 
 };
 
-namespace olive {
-/**
- * @brief Core object accessible from anywhere in the code
- *
- * Use this to access Core functions.
- */
-extern Core core;
-}
-
 #endif // CORE_H