Pagepool item and url accessors

Currently there is no way to query PagePool about which items/URLs are being managed. This change exposes two properties: items and urls which allow users finer-grained control over the PagePool. For instance, instead of clearing the entire pool, users may want to iterate through and selectively delete pages based on the URL or item.

Additional changes include migrating to QQmlComponent::createWithInitialProperties instead of wrapping object-creation inside a beginCreate/completeCreate pair, and a bug fix for an issue where items were not being added to m_urlForItem.