Add progress indicator

[mivoligo]

I like Beru and use it daily on bq phone. One feature I miss from FBReader is the indication where I am in the book at the moment. I have no idea how many pages are still left to read.
Below is a link to the screenshot from FBReader. You can see it has the "progress bar" at the bottom of the page. You can also notice there are indications for chapters there. Not sure if it's possible to do in Beru but it'd be a great addition at least for me. :)

https://lh3.ggpht.com/aYkU3ftW0Iovbpr1P_yNxl0tXQ_Joci7ZlRpbHlPecTkA68xICwGro0n-AgnfwqCwg=h900

[rschroll]

It's certainly possible, but it takes a certain amount of guesswork.

The basic problem is getting a page count for the whole book. Monocle,
the library I'm using here, doesn't give you that because it only lays
out individual components as needed. This is a sensible choice from a
performance standpoint, but means we can't say we're on page 137 of
241, for example.

I'm not convinced that we need that specificity, though. We do have a
percentage for the current component, and we can estimate the relative
sizes of the other components from their file sizes, so we can pretty
accurately say you're about 57% of the way through the book.

We do have an accurate page count for the current component, which
often, but not always, corresponds to the chapter. So we can say
you're on page 8 of 12 of the component, which is useful if you're
trying to figure out whether you can finish the current chapter before
$activity. But the chapter-component correspondence is not guaranteed,
so I'm a bit hesitant to put this in without having some sort of sanity
checking.

These issues are discussed further in #40 and #77, in slightly
different contexts.