feat: group OpenFX plugins under dedicated "OpenFX" category with sub-groups

All OpenFX plugins were previously hardcoded to return kCategoryUnknown,
  causing them to pile up under "Uncategorized" in the node creation menu.

  This commit introduces a two-level grouping system for OFX plugins:

  1. Add new kCategoryOpenFX top-level category
     - Node::CategoryID enum extended with kCategoryOpenFX
     - PluginNode::Category() now returns {kCategoryOpenFX}
     - Node::GetCategoryName() returns "OpenFX"

  2. Add secondary sub-grouping support
     - Node base class gains virtual SubCategory() method
     - PluginNode implements SubCategory() backed by sub_category_ member
     - sub_category_ is set in the constructor from the plugin's OFX context:
         Filter     → "Filter"
         Generator  → "Generator"
         Transition → "Transition"
         others     → "General"

  3. Update NodeFactory::CreateMenu()
     - When a node belongs to kCategoryOpenFX and provides a non-empty
       SubCategory(), creates a second-level submenu under "OpenFX"
     - Nodes without a sub-category are placed directly in the top menu

  Expected menu layout:
    OpenFX
      ├── Filter
      │     ├── ColorCorrect
      │     └── ...
      ├── Generator
      ├── Transition
      └── General

  All 4 test suites pass.

app/node/factory.cpp

@@ -128,25 +128,40 @@ Menu *NodeFactory::CreateMenu(QWidget *parent, bool create_none_item,
 		// Make sure nodes are up-to-date with the current translation
 		n->Retranslate();
 
-		Menu *destination = nullptr;
-
 		QString category_name = Node::GetCategoryName(
 			n->Category().isEmpty() ? Node::kCategoryUnknown :
 									  n->Category().first());
 
-		// See if a menu with this category name already exists
+		// Find or create top-level category menu
+		Menu *top_menu = nullptr;
 		QList<QAction *> menu_actions = menu->actions();
 		foreach (QAction *action, menu_actions) {
 			if (action->menu() && action->menu()->title() == category_name) {
+				top_menu = static_cast<Menu *>(action->menu());
+				break;
+			}
+		}
+		if (!top_menu) {
+			top_menu = new Menu(category_name, menu);
+			menu->InsertAlphabetically(top_menu);
+		}
+
+		// Determine final destination (support secondary grouping)
+		Menu *destination = top_menu;
+		QString sub = n->SubCategory();
+		if (!sub.isEmpty() &&
+			n->Category().contains(Node::kCategoryOpenFX)) {
+			QList<QAction *> sub_actions = top_menu->actions();
+			foreach (QAction *action, sub_actions) {
+				if (action->menu() && action->menu()->title() == sub) {
 					destination = static_cast<Menu *>(action->menu());
 					break;
 				}
 			}
-
-		// Create menu here if it doesn't exist
-		if (!destination) {
-			destination = new Menu(category_name, menu);
-			menu->InsertAlphabetically(destination);
+			if (destination == top_menu) {
+				destination = new Menu(sub, top_menu);
+				top_menu->InsertAlphabetically(destination);
+			}
 		}
 
 		// Add entry to menu

app/node/node.cpp

@@ -2295,6 +2295,8 @@ QString Node::GetCategoryName(const CategoryID &c)
 		return tr("Transition");
 	case kCategoryProject:
 		return tr("Project");
+	case kCategoryOpenFX:
+		return tr("OpenFX");
 	case kCategoryTime:
 		return tr("Time");
 	case kCategoryUnknown:

app/node/node.h

@@ -101,6 +101,7 @@ public:
 		kCategoryTransition,
 		kCategoryDistort,
 		kCategoryProject,
+		kCategoryOpenFX,
 
 		kCategoryCount
 	};
@@ -177,6 +178,12 @@ public:
    */
 	virtual QVector<CategoryID> Category() const = 0;
 
+	/**
+	 * @brief Return a sub-category string for secondary grouping
+	 *        within the primary category (e.g. "Filter" under "OpenFX").
+	 */
+	virtual QString SubCategory() const { return QString(); }
+
 	/**
    * @brief Return a description of this node's purpose (optional for subclassing, but recommended)
    *

app/node/plugins/Plugin.cpp

@@ -250,6 +250,18 @@ olive::plugin::PluginNode::PluginNode(
 	OFX::Host::ImageEffect::Instance *plugin)
 {
 	plugin_instance_=plugin;
+
+	const std::string &ctx = plugin_instance_->getContext();
+	if (ctx == kOfxImageEffectContextFilter) {
+		sub_category_ = tr("Filter");
+	} else if (ctx == kOfxImageEffectContextGenerator) {
+		sub_category_ = tr("Generator");
+	} else if (ctx == kOfxImageEffectContextTransition) {
+		sub_category_ = tr("Transition");
+	} else {
+		sub_category_ = tr("General");
+	}
+
 	bool has_texture_input = false;
 	QHash<QString, QString> group_labels;
 	QHash<QString, QString> page_labels;
@@ -505,7 +517,12 @@ QString olive::plugin::PluginNode::Name() const
 
 QVector<olive::Node::CategoryID> olive::plugin::PluginNode::Category() const
 {
-	return { olive::Node::kCategoryUnknown };
+	return { olive::Node::kCategoryOpenFX };
+}
+
+QString olive::plugin::PluginNode::SubCategory() const
+{
+	return sub_category_;
 }
 
 QString olive::plugin::PluginNode::Description() const

app/node/plugins/Plugin.h

@@ -38,6 +38,7 @@ public:
 	QString Name() const override;
 	QString id() const override;
 	QVector<CategoryID> Category() const override;
+	QString SubCategory() const override;
 	QString Description() const override;
 
 	void AddPushButton();
@@ -76,6 +77,9 @@ public:
    */
 	virtual void GenerateFrame(FramePtr frame, const GenerateJob &job) const;
 
+private:
+	QString sub_category_;
+
 public slots:
 	void pushButtonClicked(QString name);