Instead of marking annotations as orphans after the anchoring timeout
expires, instead display them in the Annotations tab but with a visual
indicator that they have not yet anchored. This avoids showing
annotations that are still anchoring in the Orphans tab, which can be
confusing when it happens in long PDF documents with many annotations.

 * Add an `$anchorTimeout` flag to annotations that is set if
   annotations fail to anchor within the timeout period.

 * Display annotations with this flag set in the Annotations tab,
   on the optimistic basis that they will eventually anchor.

 * When rendering annotations with this flag set, display them with
   a quote struck through (same as orphans) to indicate that they are
   pending. The visual representation of the pending state may be
   tweaked in future.