Document wxDataViewCtrl::GetMainWindow()

This method can be useful, so make it part of the public API, similarly
to e.g. wxGrid::GetGridWindow().

See #17786.

interface/wx/dataview.h

@@ -1442,6 +1442,16 @@ public:
     virtual wxRect GetItemRect(const wxDataViewItem& item,
                                const wxDataViewColumn* col = NULL) const;
 
+    /**
+        Returns the window corresponding to the main area of the control.
+
+        This is the window that actually shows the control items and may be
+        different from wxDataViewCtrl window itself in some ports (currently
+        this is only the case for the generic implementation used by default
+        under MSW).
+     */
+    wxWindow* GetMainWindow();
+
     /**
         Returns pointer to the data model associated with the control (if any).
     */