PDFViewer: Toggle prev/next page actions when page changes

These actions were not updated accordingly when one scrolled through the
document, and thus one could accidentally, for example, move to the next
page when standing on the last, which caused a crash.

This commit fixes that behavior, toggling the actions' enabled status
depending on the new page being displayed.

Userland/Applications/PDFViewer/PDFViewerWidget.cpp

@@ -37,6 +37,8 @@ PDFViewerWidget::PDFViewerWidget()
     m_viewer = splitter.add<PDFViewer>();
     m_viewer->on_page_change = [&](auto new_page) {
         m_page_text_box->set_current_number(new_page + 1, GUI::AllowCallback::No);
+        m_go_to_prev_page_action->set_enabled(new_page > 0);
+        m_go_to_next_page_action->set_enabled(new_page < m_viewer->document()->get_page_count() - 1);
     };
 
     initialize_toolbar(toolbar);