Refactor selection tracking support for Repetition

This restores the ability to track which iterations of a repetition are
selected.  It uses a chain of bidirectional bindings from the "selected"
property on each iteration in the repetition, to the "selection"
collection in the content controller.
Restore selection gestures to Repetition.

In the course of refactoring Repetition to use FRB, this restores the
"isSelectionEnabled" feature of a Repetition, which allows the selection
of an iteration to be toggled on touch or click.  The "active" property
on each iteration is bidirectionally bound to the content of the
"activeIterations" collection on the repetition.