VECTOR_PROPERTY: add non-const reference getter

GUI/Model/Descriptor/VectorProperty.h

@@ -73,10 +73,11 @@ private:
 // clang-format off
 
 //! Add a member, a getter and a setter for a VectorProperty
-#define VECTOR_PROPERTY(nameLower, nameUpper)                    \
-protected:                                                       \
-    VectorProperty m_##nameLower;                                \
-public:                                                          \
+#define VECTOR_PROPERTY(nameLower, nameUpper)                           \
+protected:                                                              \
+    VectorProperty m_##nameLower;                                       \
+public:                                                                 \
+    VectorProperty& nameLower() { return m_##nameLower; }               \
     const VectorProperty& nameLower() const { return m_##nameLower; }   \
     void set##nameUpper(const R3& v) { m_##nameLower.set(v); }