appDisplay: Move boundary page assertions

Since the function that manages the changes between pages is
goToPage, just move the assertions of page >= 0 and page < nPages
to that function

https://bugzilla.gnome.org/show_bug.cgi?id=707979
appDisplay: Change pages with page down/up keys

Add key bindings to app picker to allow change pages using
the page up/down keys.

https://bugzilla.gnome.org/show_bug.cgi?id=707979