Add a page loading indicator to the page number input

This PR contains a suggested implementation of the ideas in #3990. With this patch, a loading icon will be placed in the page number input field, but only when the currently visible page is actually loading.
I purposely left the loading indicators on the pages intact, since I believe that having a page loading indicator in the toolbar should supplement, rather than replace, the current ones.

Feedback welcome!

Fixes #3990.
Fixes [bug 829737](https://bugzilla.mozilla.org/show_bug.cgi?id=829737).