Revert "view: don't hit tracker every time the user scrolls"

This reverts commit 7bf56d19ad0a54c2b8eb1f1591c216424db92cbc.

For some reason, the author was reassigned to me. Reverting this and
pushing a new one with the correct author field.
view: don't hit tracker every time the user scrolls

Cache the number of items in the fully loaded view the first time
it's needed, and use that value for all future scrolling.

Note: if we ever support watching the library for changes, we need
to invalidate the cache.

https://bugzilla.gnome.org/show_bug.cgi?id=706025