Jump to page number from seek bar

**Context**

It can be useful to jump directly to a page number when reading a book, for example when a book references itself or has an unindexed table of contents. The page seek bar at the bottom of the reader is not precise enough and using page gallery view can be just as imprecise on slow connections.

**Existing implementations**
Kindle web reader:

GNOME's document viewer (evince):
Gmail pdf preview:

Sorry if this feature or request already exists. I couldn't find either of them. I also wasn't sure if this should be submitted via launchpad.