Fix removing and inserting pages in wxToolbook

Removing a page from wxToolbook could result in crashes due to
dereferencing the now invalid page index. Fix this by not assuming that
the page index is the same as its tool ID, but adding functions to
explicitly map one to the other.

Also fix inserting pages into wxToolbook, which worked as appending them
before.

Closes https://github.com/wxWidgets/wxWidgets/pull/1042

Closes #18275.

include/wx/toolbook.h

@@ -123,6 +123,14 @@ private:
     // common part of all constructors
     void Init();
 
+    // returns the tool identifier for the specified page
+    int PageToToolId(size_t page) const;
+
+    // returns the page index for the specified tool ID or
+    // wxNOT_FOUND if there is no page with that tool ID
+    int ToolIdToPage(int toolId) const;
+
+
     wxDECLARE_EVENT_TABLE();
     wxDECLARE_DYNAMIC_CLASS_NO_COPY(wxToolbook);
 };