Ctrl+scrolling with a mouse wheel zoomed in the opposite direction
than expected: scrolling up zoomed out and scrolling down zoomed in,
which is inconsistent with other GNOME apps such as Loupe and
Nautilus.
Negate the scroll delta in both smooth (touchpad) and discrete
(mouse wheel) scrolling cases, so scrolling up now zooms in.
Fixes#1633
Part-of: <https://gitlab.gnome.org/GNOME/gnome-calendar/-/merge_requests/815>
