Improve customization of Login Screen

[michaelwechner]

One can currently customize the login screen per realm according to

http://127.0.0.1:8080/yanel/yanel-website/en/documentation/security/login-screen-customization.html

but this is rather limited. It would be helpful to have the additional functionality:

• out of the box i18n support
• out of the box XInclude support
• Being able to use multiple XSLTs in a row

I guess it would be best to enhance

src/webapp/src/java/org/wyona/yanel/servlet/security/impl/DefaultWebAuthenticatorImpl.java

such that it would be using the BasicXMLResource as an overwritable global resource.