nodes: use defined categories rather than arbitrary string based

app/node/math/math/math.cpp

@@ -61,9 +61,9 @@ QString MathNode::id() const
   return QStringLiteral("org.olivevideoeditor.Olive.math");
 }
 
-QString MathNode::Category() const
+QList<Node::CategoryID> MathNode::Category() const
 {
-  return tr("Math");
+  return {kCategoryMath};
 }
 
 QString MathNode::Description() const

app/node/math/math/math.h

@@ -34,7 +34,7 @@ public:
 
   virtual QString Name() const override;
   virtual QString id() const override;
-  virtual QString Category() const override;
+  virtual QList<CategoryID> Category() const override;
   virtual QString Description() const override;
 
   virtual void Retranslate() override;