Port to event controllers
As preparation for porting to GTK4, we can adopt event controllers to stop using the GtkWidget::event signal.
This Merge Request ports the whole codebase with 2 notable exceptions:
- The scroll events in NautilusFilesView, for zooming, because GtkEventControllerScroll doesn't return a boolean in GTK3. I have a patch for this but it's GTK4-only.
- The canvas view, which is obsolete and not expected to be ported to GTK4 (see #319 (closed))