movements: add whole line page movements

When moving by ctrl+f/b or page-up/down with linewise mode, we would drift
from whole lines pretty quickly. This tries a bit harder to keep the line
selection as whole lines.

It could still use some work at the file edge boundaries, but this is
clearly better than where we were.

https://bugzilla.gnome.org/show_bug.cgi?id=766087