PDFViewer: Update the page number when scrolling between pages

Author: https://github.com/FalseHonesty
Commit: 4f0a75224c
Pull-request: https://github.com/SerenityOS/serenity/pull/8173

Userland/Applications/PDFViewer/PDFViewer.h

@@ -47,6 +47,8 @@ public:
     ALWAYS_INLINE const RefPtr<PDF::Document>& document() const { return m_document; }
     void set_document(RefPtr<PDF::Document>);
 
+    Function<void(u32 new_page)> on_page_change;
+
 protected:
     PDFViewer();