nodeparam: implemented parameters that allow arbitrary properties to be set

for UI representations

Allows properties to be set without adding explicit extra members to NodeInputs.
Crucially this means parameters like slider value representations (percent,
decibel, etc.) can be set from NodeInputs now.

app/node/input.h

@@ -216,19 +216,35 @@ public:
    */
   void set_is_keyframable(bool k);
 
-  const QVariant& minimum() const;
-  bool has_minimum() const;
-  void set_minimum(const QVariant& min);
-
-  const QVariant& maximum() const;
-  bool has_maximum() const;
-  void set_maximum(const QVariant& max);
-
   /**
    * @brief Copy all values including keyframe information and connections from another NodeInput
    */
   static void CopyValues(NodeInput* source, NodeInput* dest, bool include_connections = true);
 
+  /**
+   * @brief Set an arbitrary property on this input to influence a UI representation's behavior
+   *
+   * NodeInputs also utilize QObject's property key/value system for arbitrary properties that can influence the UI
+   * representation's behavior.
+   *
+   * Currently supported properties:
+   *
+   * - `min` - For any numeral type represented with a slider, prevents values going BELOW this number
+   * - `max` - For any numeral type represented with a slider, prevents values going ABOVE this number
+   * - `view` - For any numeral type represented with a slider, shows number either as `db`, `percent`, or `normal`
+   */
+  void set_property(const QString& key, const QVariant& value);
+
+  /**
+   * @brief Retrieve a property (or an empty QVariant if it hasn't been set)
+   */
+  QVariant get_property(const QString& key) const;
+
+  /**
+   * @brief Return whether a certain property has been set or not
+   */
+  bool has_property(const QString& key) const;
+
   QVector<QVariant> split_normal_value_into_track_values(const QVariant &value) const;
 
   QVariant combine_track_values_into_normal_value(const QVector<QVariant>& split) const;
@@ -242,6 +258,8 @@ signals:
 
   void KeyframeRemoved(NodeKeyframePtr key);
 
+  void PropertyChanged(const QString& s, const QVariant& v);
+
 protected:
   virtual void LoadInternal(QXmlStreamReader* reader, QHash<quintptr, NodeOutput*>& param_ptrs, QList<SerializedConnection> &input_connections, QList<FootageConnection>& footage_connections, const QAtomicInt* cancelled);
 
@@ -325,24 +343,9 @@ private:
   bool keyframing_;
 
   /**
-   * @brief Sets whether this param has a minimum value or not
+   * @brief Internal properties variable
    */
-  bool has_minimum_;
-
-  /**
-   * @brief Internal minimum value
-   */
-  QVariant minimum_;
-
-  /**
-   * @brief Sets whether this param has a maximum value or not
-   */
-  bool has_maximum_;
-
-  /**
-   * @brief Internal maximum value
-   */
-  QVariant maximum_;
+  QHash<QString, QVariant> properties_;
 
 private slots:
   /**