make QCustomPlot compatible with Qt6.2+ (qt6)

Cf the discussion here: https://www.qcustomplot.com/index.php/support/forum/2380

qcustomplot.h

@@ -156,26 +156,8 @@ class QCPPolarGraph;
   
   It provides QMetaObject-based reflection of its enums and flags via \a QCP::staticMetaObject.
 */
-#ifndef Q_MOC_RUN
 namespace QCP {
-#else
-class QCP { // when in moc-run, make it look like a class, so we get Q_GADGET, Q_ENUMS/Q_FLAGS features in namespace
-  Q_GADGET
-  Q_ENUMS(ExportPen)
-  Q_ENUMS(ResolutionUnit)
-  Q_ENUMS(SignDomain)
-  Q_ENUMS(MarginSide)
-  Q_FLAGS(MarginSides)
-  Q_ENUMS(AntialiasedElement)
-  Q_FLAGS(AntialiasedElements)
-  Q_ENUMS(PlottingHint)
-  Q_FLAGS(PlottingHints)
-  Q_ENUMS(Interaction)
-  Q_FLAGS(Interactions)
-  Q_ENUMS(SelectionRectMode)
-  Q_ENUMS(SelectionType)
-public:
-#endif
+    Q_NAMESPACE
 
 /*!
   Defines the different units in which the image resolution can be specified in the export
@@ -318,6 +300,20 @@ enum SelectionType { stNone                ///< The plottable is not selectable
                      ,stMultipleDataRanges ///< Any combination of data points/ranges can be selected
                     };
 
+  Q_ENUM_NS(ExportPen)
+  Q_ENUM_NS(ResolutionUnit)
+  Q_ENUM_NS(SignDomain)
+  Q_ENUM_NS(MarginSide)
+  Q_FLAG_NS(MarginSides)
+  Q_ENUM_NS(AntialiasedElement)
+  Q_FLAG_NS(AntialiasedElements)
+  Q_ENUM_NS(PlottingHint)
+  Q_FLAG_NS(PlottingHints)
+  Q_ENUM_NS(Interaction)
+  Q_FLAG_NS(Interactions)
+  Q_ENUM_NS(SelectionRectMode)
+  Q_ENUM_NS(SelectionType)
+
 /*! \internal
   
   Returns whether the specified \a value is considered an invalid data value for plottables (i.e.
@@ -386,15 +382,7 @@ Q_DECLARE_OPERATORS_FOR_FLAGS(QCP::AntialiasedElements)
 Q_DECLARE_OPERATORS_FOR_FLAGS(QCP::PlottingHints)
 Q_DECLARE_OPERATORS_FOR_FLAGS(QCP::MarginSides)
 Q_DECLARE_OPERATORS_FOR_FLAGS(QCP::Interactions)
-Q_DECLARE_METATYPE(QCP::ExportPen)
-Q_DECLARE_METATYPE(QCP::ResolutionUnit)
-Q_DECLARE_METATYPE(QCP::SignDomain)
-Q_DECLARE_METATYPE(QCP::MarginSide)
-Q_DECLARE_METATYPE(QCP::AntialiasedElement)
-Q_DECLARE_METATYPE(QCP::PlottingHint)
-Q_DECLARE_METATYPE(QCP::Interaction)
-Q_DECLARE_METATYPE(QCP::SelectionRectMode)
-Q_DECLARE_METATYPE(QCP::SelectionType)
+//no need to use Q_DECLARE_METATYPE on enum since Q_ENUM_NS adds enum as metatype automatically
 
 /* end of 'src/global.h' */