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/value.h

@@ -197,6 +197,10 @@ public:
 		kBinary,
 
 		/**
+		 *Push Button
+		 */
+		kPushButton,
+		/**
      * End of list
      */
 		kDataTypeCount