Fix wxTreeListCtrl under wxOSX

Postpone resetting indent in wxOSX wxDataViewCtrl to avoid always removing,
and hence never showing the expanders at all, for wxTreeListCtrl whose model
starts its life as a list but becomes a tree as soon as any items with
children are added to it.

By postponing the call to IsListModel() until the next resize, we give the
model the time it needs to decide what it's going to be, while still removing
the unnecessary indent if there is no need for it.

Closes #17409.

include/wx/osx/cocoa/dataview.h

@@ -534,6 +534,8 @@ private:
     wxCocoaOutlineDataSource* m_DataSource;
 
     wxCocoaOutlineView* m_OutlineView;
+
+    bool m_removeIndentIfNecessary;
 };
 
 #endif // _WX_DATAVIEWCTRL_COCOOA_H_