appDisplay: Remove redundant checks for pageNumber

We already do these at the beginning of the function.

https://bugzilla.gnome.org/show_bug.cgi?id=729064
appDisplay: Fix pages getting "stuck" under touch

We often call goToPage like:

  this.goToPage(this._currentPage - 1);

(or + 1). During panning, these are based on the velocity values
of the gesture action. If we're already on the first or last page
and the panning ends, it's possible to get goToPage that's either
-1 or greater than the last page.

During normal usage, this isn't a problem, since the Y values will
be correct, always. But when panning, the Y values stick to the
finger, and thus if we return early, we won't snap to the exact page,
making it seem like things get "stuck".

Fix this the simple way by clamping to the correctly-ranged values
of pageNumber

https://bugzilla.gnome.org/show_bug.cgi?id=729064
appDisplay: Make page panning smoother on touch

Currently, our logic for page panning isn't great. If the user starts a
pan upwards and hesitates over a new page, we'll go to the *next* page
on release, since the difference is greater, but the velocity wound down
to 0.

Instead of trying to treat it like page down or scrolls, simply do the
math to find the page where the user scrolled to.

This is unfortunately broken for fast swipes, since the user doesn't get
far enough into the new page to make a difference. I'm getting the
impression we'll need a gesture recognizer for this, though, however
crude. Simple hacks I tried, like a velocity multiplier, didn't work
properly.

https://bugzilla.gnome.org/show_bug.cgi?id=729064