Help: Make history navigation work with man pages opened using help urls

This patch implements man pages opened using the help url protocol
properly getting added to the navigation history as well as enabling
the back and forward buttons in such cases.

Userland/Applications/Help/MainWidget.cpp

@@ -114,7 +114,10 @@ MainWidget::MainWidget()
                 }
                 auto const section = url.paths()[0];
                 auto const page = url.paths()[1];
-                open_url(URL::create_with_file_scheme(String::formatted("/usr/share/man/man{}/{}.md", section, page), url.fragment()));
+                auto const path = String::formatted("/usr/share/man/man{}/{}.md", section, page);
+
+                m_history.push(path);
+                open_url(URL::create_with_file_scheme(path, url.fragment()));
             } else {
                 dbgln("Bad help operation '{}' in URL '{}'", url.host(), url);
             }
@@ -266,6 +269,9 @@ ErrorOr<void> MainWidget::initialize_fallibles(GUI::Window& window)
 
 void MainWidget::open_url(URL const& url)
 {
+    m_go_back_action->set_enabled(m_history.can_go_back());
+    m_go_forward_action->set_enabled(m_history.can_go_forward());
+
     if (url.protocol() == "file") {
         m_web_view->load(url);
         m_web_view->scroll_to_top();