Check footage in use before deleteing

Initial very hacky implementation that checks if the footgae is in
use before deleting it and makes sure all refernences to it are
properly cleaned up.

Also added IsMedia() function to Node to simplify things a bit.

app/node/node.h

@@ -363,6 +363,15 @@ public:
    */
   virtual bool IsTrack() const;
 
+
+  /**
+   * @brief Returns whether this Node is a "Media" type or not
+   *
+   * You shouldn't ever need to override this since all derivatives of Media will automatically have this set to true.
+   * It's just a more convenient way of checking than dynamic_casting.
+   */
+  virtual bool IsMedia() const;
+
   /**
    * @brief The main processing function
    *