Don't let WebView get focus on BookPage

The WebView would get focus and eat up all keypresses.  The docs suggest
that these should bubble up to the page, but that's not happening for
some reason.  So we make the WebView drop focus as soon as it gets it.
Implement cover view

Based on work by Stuart Langridge.  Ref #16.

We replace the recently-read list view with this, since that's a good
fit for the visual search.  Instead of using a single GridView for both
the covers and the lists, we swap between a GridView and a ListView.
Hopefully this will make things a little less tangled.  The positioning
of the GridView runs into the same sorts of problems as in #17, but the
work-around actually works in this case.

This includes a basic cover generator for books that don't have any.
Note that the title and author are added in QML, because doing flowed
text in SVG is difficult.  (Inkscape uses its own extension to do this,
but Qt doesn't recognize those elements.)
Add drop shadow to cover view

This keeps it from looking too flat, but it could be computationally
expensive.
Align scrollbar for gridview to Page

The margin on the gridview pushed the scrollbar off the edge a bit.
Update database to hold higher res copy of covers

We keep the low res copies around and continue to use them in the
ListViews, since scaling the high res copies down into those icons
didn't work well.

Closes #16.