Added wxBookCtrlBase::FindPage().

Add helper function to find the index of the given page.

Closes #15125.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73734 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/bookctrl.h

@@ -215,6 +215,9 @@ public:
         }
     }
 
+    // return the index of the given page or wxNOT_FOUND
+    int FindPage(const wxWindow* page) const;
+
     // hit test: returns which page is hit and, optionally, where (icon, label)
     virtual int HitTest(const wxPoint& WXUNUSED(pt),
                         long * WXUNUSED(flags) = NULL) const