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.
Search for books recursively

It's easier for listDir to return absolute filenames now, since we'll
have to split filenames in subdirectories anyway.  Closes #30.
Add refresh button to LocalBooks

Closes #21.
Specify Qt5 in launcher script

Closes #41.
Avoid two "Back" buttons in the toolbar

Instead, label the one to move back in the page stack "Library".  Closes #36.
Get color for line height slider from initial styles

Closes #45.
Use iconSource instead of icon

Closes #43.
Only clear history if it exists

The history property of the BookPage is no longer being created before
onVisibleChanged is called for the first time.  Dunno why.  Closes #42.
Lock toolbars while popups pointing to them are open

Closes #46, refs #49.
Don't handle blank messages

For some reason, we're now getting a blank message when first loading
the BookPage.
Set EpubReader.zip to NULL during initialization

Closes #52.
Revert "Lock toolbars while popups pointing to them are open"

This reverts commit 795c9ed2ed446fe52b56308a087e0a6a9ac2d8b3.  It was
causing the contents of the page to jump up when the toolbar was locked.
Open #46.
Hold toolbar open by adjusting timeout

The MainView adjusts the layout when the toolbar is locked open, which
we don't want.  So instead, we turn off the timeout when the popover is
open, so the toolbar will stay open until the popover is closed.  In
anticipation of the new header, we do this only on the book page.
Closes #46.
Set window title when reading book

There doesn't seem to be a good way to override the window title all the time, but for a single page that will only be at the top of the PageStack, it's possible.  The window title is most annoying for the BookPage, so set it for that one.  Ref. #48.
Use the new header, except on the BookPage

Refs #49.
Move "Reload" into settings dialog

This avoids the drop-down menu for actions on the Library page.  Refs #49.