page: Replace implementing GtkScrollable by a property

GtkScrollable was abused, as it isn't designed to access widgets which
can fully scroll on their own, but for widgets you can put in a scrolled
window. This stops making pages implement it and instead make them
provide their scrolled window's vertical ajustment via a property.

This makes the code more correct and much simpler.
3 jobs for scrollable in 6 minutes and 10 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #1342632
debian-stable-x86_64

00:02:07

passed #1342631
fedora-x86_64

00:06:10

manual #1342633
flatpak allowed to fail manual
flatpak bundle