Menu interactions

Updates since last pull request
On a page, the number of sentences doesn't necessarily correspond with the last sentence number. This is because only the first page of a chapter starts with sentence 1, while subsequent pages start wherever the previous page left off. For example, page 1 may have three sentences: 1, 2, and 3, while page 2 may also have three sentences labeled 4, 5, and 6. We need the last sentence number so that we access the solution steps for the correct sentence and not end up with solution steps for sentences from the previous page.