Browser: Go back/forward when pressing back/forward mouse buttons

This currently doesn't work when running Serenity through QEMU, as it
doesn't pass the side button events over to Serenity due to some bug or
missing feature.

Userland/Applications/Browser/Tab.cpp

@@ -445,6 +445,16 @@ Tab::Tab(BrowserWindow& window)
         load(url);
     };
 
+    view().on_back_button = [this] {
+        if (m_history.can_go_back())
+            go_back();
+    };
+
+    view().on_forward_button = [this] {
+        if (m_history.can_go_forward())
+            go_forward();
+    };
+
     m_tab_context_menu = GUI::Menu::construct();
     m_tab_context_menu->add_action(GUI::CommonActions::make_reload_action([this](auto&) {
         reload();

Userland/Libraries/LibWebView/OutOfProcessWebView.cpp

@@ -174,6 +174,12 @@ void OutOfProcessWebView::mousedown_event(GUI::MouseEvent& event)
 void OutOfProcessWebView::mouseup_event(GUI::MouseEvent& event)
 {
     enqueue_input_event(event);
+
+    if (event.button() == GUI::MouseButton::Backward && on_back_button) {
+        on_back_button();
+    } else if (event.button() == GUI::MouseButton::Forward && on_forward_button) {
+        on_forward_button();
+    }
 }
 
 void OutOfProcessWebView::mousemove_event(GUI::MouseEvent& event)

Userland/Libraries/LibWebView/OutOfProcessWebView.h

@@ -111,6 +111,8 @@ public:
     Function<Gfx::IntRect()> on_maximize_window;
     Function<Gfx::IntRect()> on_minimize_window;
     Function<Gfx::IntRect()> on_fullscreen_window;
+    Function<void()> on_back_button;
+    Function<void()> on_forward_button;
 
 private:
     OutOfProcessWebView();