Previously, the IdeCodeIndexEntries API required generating content on the main
thread (or rather, accessing it synchronously with a generator-like API).

This instead creates an async API to get the next chunk of items with a default
implementation to call that sync API on the main thread.

The Vala and Clang indexers have been ported to the Async API to batch all the
results in a single call.

This allows less work to be done on the main thread (other than building the
index GVariant content files).

A new IdeCodeIndexerEntryBuilder API has been created to create entry
instances. This also allowed us to make IdeCodeIndexerEntry not an object
anymore, but a boxed type.  This should be a little bit faster by avoiding all
those property creations.