feature/ajax-loading-indicator -> refactored for extensibility

Hey - nice idea!

I split up the spinner impl from being so tightly coupled w the component impl, since not every wants the same loading indicator graphic.

Moved property spinner loading from the resource itself to the `cq:Component` node `ajaxLoadingIndicator` that takes a CSS Class. If nothing is set, spinner is disabled. If we really want authors to be able to dictate if the spinner shows, we can add this back to the resource, but IMO ultimately the spinner impl choice should be on the `cq:Component` itself.

Move the "default" ACS Commons loading indicator into its own clientlib `acs-commons.components.ajax.loading-indicator` .. so ppl can include this, or create their own, depending on their needs.

WDYT?