WindowServer+LibGUI: Support the 4th and 5th mouse buttons

We'll call the "Back" and "Forward" since that's what they are normally
mapped to in some other systems.

Servers/WindowServer/WindowManager.cpp

@@ -683,6 +683,10 @@ auto WindowManager::DoubleClickInfo::metadata_for_button(MouseButton button) ->
         return m_right;
     case MouseButton::Middle:
         return m_middle;
+    case MouseButton::Back:
+        return m_back;
+    case MouseButton::Forward:
+        return m_forward;
     default:
         ASSERT_NOT_REACHED();
     }