Add push button support for OFX plugins

This commit adds support for push buttons in OFX plugins by:

- Adding `kPushButton` to `NodeValue` enum
- Implementing `pushButtonClicked` slot in `PluginNode`
- Creating `NodeParamButton` widget for UI representation
- Updating `paraminstance.h` to handle push button instances
- Modifying `nodeparamviewwidgetbridge.cpp` to connect push button signals

app/node/plugins/Plugin.h

@@ -40,6 +40,7 @@ public:
 
 	void AddPushButton();
 	void AddPage();
+
 	Node *copy() const override;
 	/**
    * @brief The main processing function
@@ -73,6 +74,9 @@ public:
    */
 	virtual void GenerateFrame(FramePtr frame, const GenerateJob &job) const;
 
+public slots:
+	void pushButtonClicked(QString name);
+
 };
 
 }