Add unsaved changes prompt

When ingame, there should be a prompt of unsaved changes when accidentally tapping on another page.

So I play Calibre 2048, and I accidentally tap another page. The browser should then say "You have unsaved changes" so that I don't lose the data.

@Piotr Grochowski , it is a good idea. We will investigate to see whether it is feasible and how to implement it.