Move BrowserPage to use DownloadManager

Unfortunately, DownloadManager puts it in a place we can't access.  See
https://bugs.launchpad.net/ubuntu/+source/apparmor-easyprof-ubuntu/+bug/1384349
Only use BrowserPage if DownloadManager exists

Otherwise, launch the system browser.
Add spinner while laying out book

This can take a few seconds on the device, so it's nice to indicate that
things are happening.
Copy slider from Monocle
New flipper behavior

A tap produces an instant page turn, while swiping gives the animated
page turn shown previously.

Closes #69.
Fix translations

Also fix up the CMakeLists.txt files to avoid duplication.

Thanks to David Planella for debugging.
Splash screen

Closes #70.
Smarter swipe-to-turn behavior

Previously, the direction of the page turn was determined by where the
swipe started, and whether it completed was determined by where the
swipe ended.  Now we pay attention to the direction that the swipe
starts.  Swipe to the left to go forward and to the right to go back.
Whether the turn completes depends on the release speed.  If it's above
a threshold, it completes if its in the same direction as it started and
aborts otherwise.  If the release speed is nearly zero, it completes if
there was net movement in the initial direction.

Taps still give instant page turns, with the direction determined by the
half on which the tap occurred.  Additionally, quick swipes lasting less
than 250 ms complete without any animation, as do longer swipes that end
with a large speed and a page turn.

Closes #34.