Adjust index usage

Previously we used prev_index everywhere, which could get confusing - it's not really the "previous" index until we've moved. So, only page_changed_cb () refers to it as prev_index. Everywhere else it's called current_model_index.

In accordance with this change, we can get rid of a call to current_page() and index_of() since we already know the index of the current page.

We also now use binary search in calls to index_of() instead of a linear search.