[CSWH] Enable branding for the intermediate page

Our windowhandler.html intermediate page for the ClientSideWindowHandler is currently just white which showing 'Loading...'

We should introduce an easy way to at least set the background color or any other $$bodyattribute$$ which gets replaced via a setting we get from the ClientConfig.

That way a project can easily match the very basic look & feel of the intermediate page with it's project. For more changes, the whole windowhandler.html should be replaced with a custom version.