listitemmanager: Add trackers

listitemmanager: Add trackers

... and replace the anchor tracking with a tracker.

Trackers track an item through the list across changes and ensure that
this item (and potentially siblings before/after it) are always backed
by a GtkListItem and that if the item gets removed a replacement gets
chosen.

This is now used for tracking the anchor but can also be used to add
trackers for the cursor later.