API: Reinstate "gio-desktop-app-info-lookup" extension point

Removing an extension point is an API and ABI break. In particular, it
causes (older) gvfs versions to fail loading with a linkage error from
ld which in turn makes the desktop unusable.
So this reinstate the extension point and API provided by it, but
deprecates and does not use it. So no functionality is changed.

This reverts parts of commit 9b262f1c.

Complaints-Also-To: Ryan Lortie <desrt@desrt.ca>