XSIframe linker compatible script/css loading

Greetings to Norway, just returned from a skiing trip from your beautiful mountains! Here is some stuff coded coded on that trip (during car trips through sweden and on one rainy day in cabin).

The new "super dev mode compatible" XSIframe linker in GWT don't allow script tags in module description files. Scripts defined in modules are also hard to override.

This change modifies the core module so that necessary script and css is loaded compiled into GWT module with resource bundle. This technique works with XSIFrame linker (and e.g. with Vaadin). The technique also allows end users to override Leaflet script they use and creates less server requests (faster startup).

Other modules still don't work with similar technique, but I'm not interested about them in my current project.

cheers,
matti