Add Html\Attributes class, implement in some service providers

- Consolidate attribute handling code in Xoops\Html\Attributes
- implement change in Xoops\Form\Element
- implement an Html\Img class to encasulate img tag generation
- implement Img and Attributes in Qrcode and Thumbnail service providers
- add CountryFlags service and provider in system module
- update examples in codex module

Note: clear caches or update system module after installing these changes