Replace Gtk.EventControllerLegacy with Gtk.EventControllerScroll

[ryonakano]

Problem

Originally from #295 (comment)

We're using Gtk.EventControllerLegacy instead of Gtk.EventControllerScroll but it's said as a last resort in the valadoc:

https://valadoc.org/gtk4/Gtk.EventControllerLegacy.html

Proposal

Replace Gtk.EventControllerLegacy with Gtk.EventControllerScroll.

Prior Art (Optional)

No response