Add an option to navigate back to Contents in Ebook Reader

[dhruvmisra]

In the Ebook Reader, once we navigate to a page of a book, there is no option to go back to Contents (Index). The only 2 ways are either by closing the entire activity and opening a new one, or by pressing the previous page button enough times to navigate back.

Problematic Scenario:
A user accidentally clicked on the wrong page number in contents.

Potential solution:
Adding a button in the toolbar to navigate back to contents or the start of the book.

[llaske]

Why not.

[llaske]

Fixed in #652