webui: layer for standalone pages which use WebUI framework
    
    Current compiled Web UI layer (app.js) contains every FreeIPA plugin and
    not just the UI framework. It's not possible to start just a simple facet.
    
    This commit creates a basis for a layer (core.js) which contains only
    framework code and not entity related code.
    
    Reviewed-By: Endi Sukma Dewata <edewata@redhat.com>