nodes: implemented infrastructure for caching any node

Not accessible through UI yet, but ported Footage node to it
(previously we used a hack to ensure Footage textures were
cached, now it's a formal infrastructure).

app/node/node.h

@@ -756,6 +756,16 @@ public:
     folder_ = folder;
   }
 
+  bool GetCacheTextures() const
+  {
+    return cache_result_;
+  }
+
+  void SetCacheTextures(bool e)
+  {
+    cache_result_ = e;
+  }
+
   static const QString kDefaultOutput;
 
 protected:
@@ -1191,6 +1201,8 @@ private:
 
   int operation_stack_;
 
+  bool cache_result_;
+
 private slots:
   /**
    * @brief Slot when a keyframe's time changes to keep the keyframes correctly sorted by time