nodes: improved changed signal processing

Improves stability and cache reliability.

Earlier iterations were prone to skipping necessary signals (usually
leading to some sort of assert fail), particularly when track
optimizations were used. Those optimizations have been moved to the
viewer node so there's a higher degree of control over which signals
get optimized and in which ways.

app/node/node.h

@@ -304,6 +304,19 @@ public:
    */
   virtual void InvalidateCache(const TimeRange& range, NodeInput* from, NodeInput* source);
 
+  /**
+   * @brief Limits cache invalidation temporarily
+   *
+   * If you intend to do a number of operations in quick succession, you can optimize it by running
+   * this function with EndOperation().
+   */
+  virtual void BeginOperation();
+
+  /**
+   * @brief Stops limiting cache invalidation and flushes changes
+   */
+  virtual void EndOperation();
+
   /**
    * @brief Adjusts time that should be sent to nodes connected to certain inputs.
    *